Add Active Job's assertions to ActiveSupport::TestCase

Since we should test job queuing/execution inside other components of
the application like models and controllers, we should make the custom
assertions provided by `ActiveJob::TestHelper` available directly from
`ActiveSupport::TestCase`.

Thus, we don't need the `ActiveJob::TestCase` class anymore so we can
remove it and avoid relying on it when we generate a job's test.
Add tests for the job generator

Let's add tests to ensure that this generator get the works done when we
are calling it along the lines of the other ones.