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.