These specs were using a code snippet copied from a GitHub issue
comment, but there are some extra subtleties if we want to be able to
use two specs that set the ActiveJob adapter to Sidekiq in the same run.
Also move this to spec metadata so we can easily reuse it elsewhere if
we need to.

The reason we need this anyway is because we can't easily set a non-test
adapter for ActiveJob in tests: either for a specific test, or
globally. (Request specs will get set back to the test adapter.)