`subclasses` get only child classes.
Therefore, if create a job common parent class as `ApplicationJob`,
inherited class does not get properly.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

test.

This PR adds a method called `queue_adapter_for_test` to
`ActiveJob::TestHelper`. This method is expected to provide the queue
adapter to be used for jobs under test. It maintains the current
behaviour by defaulting to an instance of
`ActiveJob::QueueAdapter::TestAdapter`. Tests that include
`ActiveJob::TestHelper` or extend from `ActiveJob::TestCase` can provide
a custom queue adapter by overriding `queue_adapter_for_test` in their
class.

Instance methods can be defined in the module itself

[CI skip]

[Robin Dupret]

This allows different `queue_adapters` to be used in each `ActiveJob`
class heirarchy. Previously, all subclasses used a single global queue
adapter.

The filter was set on the pseudo-global TestAdapter but not restored to
its original value.

See e818f657

Since `ActiveJob::TestHelper` globally sets
`ActiveJob::Base.queue_adapter` on setup, there is no benefit in
instantiating a new `TestAdapter` per tests. The original rationale was
to allow parallel tests to run without interference, but since they'd
all mutate the global `ActiveJob::Base.queue_adapter`, that was never
realized.

type.

This allows specific jobs to be tested, while preventing others from
being performed unnecessarily.

Example:

    def test_hello_job
      assert_performed_jobs 1, only: HelloJob do
        HelloJob.perform_later('jeremy')
        LoggingJob.perform_later
      end
    end

An array may also be specified, to support testing multiple jobs.

Example:

    def test_hello_and_logging_jobs
      assert_nothing_raised do
        assert_performed_jobs 2, only: [HelloJob, LoggingJob] do
          HelloJob.perform_later('jeremy')
          LoggingJob.perform_later('stewie')
          RescueJob.perform_later('david')
        end
      end
    end

Fixes #18802.

Trim space and document :only option.

With the option, assert_enqueued_jobs will check the number of times a specific kind of job is enqueued.

1. The :test adapter wasn't going through a full cycle of
serialize/deserialize when performing jobs. Now it does
2. Regular AJ tests were not run for the :test adapter. Now they are
3. ActiveJob::TestHelper uses assert_valid_keys but doesn’t requires
the file that implements that method. Now it does

It will set proper queue_adapter state required by assert_performed_jobs assertions.

Also the block version of assert_performed_jobs will make sure this
state is respected.

Fixes #17684

[ci skip]

- Fix mentioned shortcut, to what the shortcut actually is, and that it accepts blocks for `assert_no_enqueued_jobs` and `assert_no_performed_jobs` test helpers.
- args => arguments when used in actual docs.

[ci skip]