Since using a browser is only for selenium it doesn't really make sense
to have a separate class for handling it there. This brings a lot of the
if/else out of the main SystemTestCase class and into the Driver class
so we can abstract away all that extra work.

`Capybara::RackTest::Driver` does not support taking screenshots. If call
`#save_screenshot` on `Capybara::RackTest::Driver` will raise the error.

```ruby
Error:
UsersTest#test_visiting_the_index:
Capybara::NotSupportedByDriverError: Capybara::Driver::Base#save_screenshot
```

To prevent errors, if driver does not support screenshot, do not call it.

In ca324a0d the flash middleware was effectively removed by its
constructor returning the app it was passed and the `commit_flash`
call was moved to the `ActionController::Metal#dispatch` method.
This broke any redirect routes that modified the flash because the
redirect happens before `dispatch` gets called.

To fix it, this commit adds a `commit_flash` call in the `serve`
method of `ActionDispatch::Routing::Redirect`.

Fixes #27992.

A gzip file has a checksum and length for the decompressed data in its
footer which isn't checked by just calling Zlib::GzipReader#read.
Calling Zlib::GzipReader#close must be called after reading to the end
of the file causes this check to be done, which is done by
Zlib::GzipReader.wrap after its block is called.

Previously, `driven_by` would change the Capybara configuration when
the test case is loaded, and having multiple test classes with different
`driven_by` configs would fail as the last loaded would be effective.

Since a `direct` url helper block is evaluated using `instance_exec`
then methods that are available in the instance context can be
accessed, e.g. the params object in a controller action or view.

This wasn't clear from the example so expand on that point and add
a test case for this situation.

Use a separate method called `resolve` for the custom polymorphic
mapping to clarify the API.

Allow the use of `direct` to specify custom mappings for polymorphic_url, e.g:

  resource :basket
  direct(class: "Basket") { [:basket] }

This will then generate the following:

  >> link_to "Basket", @basket
  => <a href="/basket">Basket</a>

More importantly it will generate the correct url when used with `form_for`.

Fixes #1769.

Allow the definition of custom url helpers that will be available
automatically wherever standard url helpers are available. The
current solution is to create helper methods in ApplicationHelper
or some other helper module and this isn't a great solution since
the url helper module can be called directly or included in another
class which doesn't include the normal helper modules.

Reference #22512.

* Override integration test default host

Integration tests automatically set the default host to
'http://example.com'. This works fine for integration tests because they
are not real browser sessions, but doesn't work fine for system tests
because they are real browser sessions.

We can override this by setting the `host!` in `before_setup. The
`Capybara.always_include_port` will allow the test to look at
`127.0.0.1:port capybara picks` and properly redirect the test.

Any application can override this by setting the `host!` in
their system test helper. Generally though, applications are going to be
using localhost.

In this commit I also moved the setup and teardown into their own module
for tidiness.

* Move teardown settings into system test case

These configuration options can be put into the system test case file
instead of the generated system tests helper file. This is an
implementation detail and therefore shouldn't be generated with the
template.

We only want the file name to include the word `failures` if it failed,
not any time the user wants to take a screenshot during a test run.

* Move system tests back into Action Pack
* Rename `ActionSystemTest` to `ActionDispatch::SystemTestCase`
* Remove private base module and only make file for public
`SystemTestCase` class, name private module `SystemTesting`
* Rename `ActionSystemTestCase` to `ApplicationSystemTestCase`
* Update corresponding documentation and guides
* Delete old `ActionSystemTest` files

```
go get -u github.com/client9/misspell/cmd/misspell
misspell  -w -error -source=text .
```

Mixing Rack::Test::Methods into ActionDispatch::IntegrationTest seems
to trigger Ruby bug 13107[1]. By using our methods instead of rack-test
we shouldn't trigger the bug in Forwardable.

[1]: https://bugs.ruby-lang.org/issues/13107

existing_acrnoyms -> existing_acronyms

(I personally prefer writing one string in one line no matter how long it is, though)

This reverts commit 5eff7a9c, reversing
changes made to 5f03172f.

Reason: It caused a regression. The test case is on the PR.

    ActiveSupport::Duration.parse('P3Y') == 3.years # It should be true

Duration parsing made independent from any moment of time:
Fixed length in seconds is assigned to each duration part during parsing.

Changed duration of months and years in seconds to more accurate and logical:

 1. The value of 365.2425 days in Gregorian year is more accurate
    as it accounts for every 400th non-leap year.

 2. Month's length is bound to year's duration, which makes
    sensible comparisons like `12.months == 1.year` to be `true`
    and nonsensical ones like `30.days == 1.month` to be `false`.

Calculations on times and dates with durations shouldn't be affected as
duration's numeric value isn't used in calculations, only parts are used.

Methods on `Numeric` like `2.days` now use these predefined durations
to avoid duplicating of duration constants through the codebase and
eliminate creation of intermediate durations.

Regression introduced by ae291421 / 8363b879.

Previously, cookies were only updated on `GET` requests. Now we will
update the helper for all requests, as part of `process`. Added
regression tests for all available HTTP method helpers in
`ActionController::TestCase`.

Actually, private methods cannot be called with `self.`, so it's not just redundant, it's a bad habit in Ruby

Some methods were added to public API in
5b14129d and they should be not part of
the public API.

When content type header is blank we were raising an exception because
`empty?` was being called on nil.

Follow up to 79a5ea9e

`Gem.win_platform?` check if it is Windows more accurately.
Ref: https://github.com/ruby/ruby/blob/ruby_2_2/lib/rubygems.rb#L945..L952

I have been seeing people setting `Logger` instances for `config.logger`
and it blowing up on `rails/web-console` usage.

Now, I doubt many folks are manually setting `ActionView::Base.logger`,
but given that `DebugExceptions` is running in a pretty fragile
environment already, having it crash (and being silent) in those cases
can be pretty tricky to trace down.

I'm proposing we verify whether the `ActionView::Base.logger` supports
silencing before we try to do it, to save us the headache of tracing it
down.

This image has copyright that we are not giving so it is better to use
one image that we own the copyright.