ActiveJob testing improvements

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
Merge pull request #18266 from cristianbica/ajfixes

ActiveJob testing improvements