If, doing a test like this:

```
class BugTest < ActionView::TestCase
  def test_foo
    omg
  end
```

Will raise with:
```
RuntimeError: In order to use #url_for, you must include routing helpers
explicitly. For instance, `include
Rails.application.routes.url_helpers`.
```

Thats a bit confusing, as we are not calling url_for at all.

[ci skip] Modify introduction to be consistent with other guides

Closes #21121

Reload I18n.load_path in development

make generated scaffold functional tests work inside API engines

This method raises conditionally not always so we should not documment
as it always raise.

We should return when the contoller key is not present or if the
controller doesn't exist and we didn't raised an error.

Related with dc1b937d

eliminates calling `scope` in one method, pushes the other calls up one
frame.  This goes a little way towards eliminating the internal calls to
`scope`.

[ci skip] Note that each action maps to a specific CRUD operation

* set engine's route in setup
* use fixture with engine namespace

we don't really need this hash.

we need to get a grip on what `scope` actually does.  This commit
removes some of the internal calls to `scope`.  Eventually we should add
public facing methods that provide the API that `scope` is trying to
accomplish.

`prepare_params!` would raise an exception if `params` wasn't
initialized, so it must always be available.  Remove the existence
conditional from the `controller` method.

The method we called already has the conditional we need.  Just add an
else block so that we don't need two tests.

`Dispatcher` doesn't need to hold on to the defaults hash.  It only used
the hash to determine whether or not it should raise an exception if
there is a name error.  We can pass that in further up the stack and
alleviate Dispatcher from knowing about that hash.

We know in advance whether the object is a dispatcher or not, so we can
configure the Constraints object with a strategy that will call the
right method.

it isn't used.

Reference actual classes

[ci skip] Fix the AS::Callbacks terminator docs

[ci skip] Give in-depth explanation of migrations vs. seeds.rb

[ci skip] Adding a note to Action Mailer Basics documentation that Google increased its security measures

security measures so using the example for Gmail will return a “Password Incorrect” error,
and you will receive an email from Google that they blocked a sign-in attempt.  You can change
your Gmail settings or use another ESP.

I discovered this when I was testing a simple mailer example app and was just going to
use my personal Gmail account for the test.  I think it would be best to note this change
since now Gmail may not be the best option for a quick test.  I hope this saves time for other Rails
developers.  The Gmail example does show a good example of how to configure the smpt settings.

The second argument of the terminator lambda is no longer the result
of the callback, but the result lambda.

https://github.com/rails/rails/blob/3a7609e2bafee4b071fe35136274e6ccbae8cacd/activesupport/test/callbacks_test.rb#L553

replace each with each_key when only the key is needed

Using each_key is faster and more intention revealing.

Calculating -------------------------------------
                each    31.378k i/100ms
            each_key    33.790k i/100ms
-------------------------------------------------
                each    450.225k (± 7.0%) i/s -      2.259M
            each_key    494.459k (± 6.3%) i/s -      2.467M

Comparison:
            each_key:   494459.4 i/s
                each:   450225.1 i/s - 1.10x slower

The tests and methods were hard to read with `options[:options]` all
over the place. This refactoring makes the code easier to understand.

The change came out of work for moving the underlying code of controller
tests to integraiton tests.

Allow a custom dispatcher to be provided to routing.

Using the string version of the class reference is now deprecated when
referencing middleware. This should be written as a class not as a string.

Deprecation warning that this change fixes:
```
DEPRECATION WARNING: Passing strings or symbols to the middleware
builder is deprecated, please change
them to actual class references.  For example:
  "ActionDispatch::ShowExceptions" => ActionDispatch::ShowExceptions
```

`extra_keys` is a confusing variable name because it's not clear what is
"extra". This renames it to `query_string_keys` so it's clear that the
"extra" is just the query string.

We were doing extra work that could be pushed off to Integration test
and SharedRoutes. Creating an extra module isn't necessary when those
are created by their respective classes.