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.

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

This adds documentation for the Active Job API. It includes
documentation on how to configure the queue_adapter, and how to create
new jobs. It adds links to the various other sections of the Active Job
documentation.