A few have been left for aesthetic reasons, but have made a pass
and removed most of them.
Note that if the method `foo` returns an array, `foo << 1`
is a regular push, nothing to do with assignments, so
no self required.
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.
