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.

Further missing requires for Timeout exposed due to Bundler 1.14.5

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

Renames `Rails::SystemTestCase` to `ActionSystemTest` and moves it to a
gem under the Rails name.

We need to name the class `ActionSystemTestCase` because the gem expects
a module but tests themselves expect a class.

Adds MIT-LICENSE, CHANGELOG, and README for the future.

Rubocop / code climate don't like single quotes and prefer doubles.

Puma is the default webserver of Rails. Because of this it doesn't make
sense to run tests in Webkit if the default server is Puma.

Here I've refactored the webserver to be it's own standalone module so
it can be shared between Rails' selenium default driver and Capybara's
defaut drivers.

This change adds support, tests, and documentation for the screenshot
helper.

If taking screenshots is supported by the driver (for example Rack Test
doesn't support screenshots) then a screenshot will be taken if the test
fails.

This removes the useless Rack Test Driver that Rails was providing and
moves to a shim like approach for default adapters.

If someone wants to use one of the default Capybara Drivers then we will
initialize a new `CapybaraDriver` that simply sets the default driver.

Rails though is much more opinionated than Capybara and to make system
testing a "works out of the box" framework in Rails we have the
`RailsSeleniumDriver`. This driver sets defaults that Rails deems
important for selenium testing. The purpose of this is to simply add a
test and it just works.

* Adds test case test
* Adds driver adapter test
* Adds tests for capybara seleium driver (testing the settings not
actually opening the browser to test capybara w/ selenium because that
would so so so slow)
* Adds tests for rack test driver
* Adds tests for generators

Closes #28033

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

since the test names become Regexp filters, non-escaped test names cause RegexpError on isolated test via bin/test

Prevents PATH_INFO from being used to infer the request format in later
test requests when no explicit format is given.

As the request PATH_INFO may be set before a request, it can't be
deleted during pre-request scrubbing.

Fixes #27774

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

These are followups for 307065f9,
but TBH I'm personally not very much confortable with this style.
Maybe we could override assert_equal in our test_helper not to warn?

we call them only in the tests