Rely on perform_one for inline testing.

Inline testing currently overrides perform_async to simply directly call

    SomeJob.new.perform(*args)

after round-tripping *args through JSON serialization/deserialization
and setting up a fake worker.jid value.

After Sidekiq issue 1938, most (all?) other places now use execute_job
instead, which indirectly calls SomeJob.new.perform(*args).

testing.rb defines a perform_one which is one of the places where the
new execute_job method is used. It also takes care of setting
worker.jid.

This change replaces the inline testing mode from a direct
SomeJob.new.perform method call to instead:

    1. Place a job on the faked array queue, at the front of the list.
    2. Immediately call perform_one, to take the job off the front of
       the list.

This reduces the duplication around what it means to execute a job,
makes the usage of execute_job consistent across applications, and
improves inline testing's mirroring of how the job would actually be
run -- by "placing" it on a queue and then "pulling" the job off the
queue straight away.

As a bonus, any third-party gems which provide an implementation of
execute_job (such as in issue 1938) can test that their execute_job is
working by using perform_async with inline testing, instead of manually
calling perform_one in the fake testing mode or disabling Sidekiq
testing entirely.
Merge pull request #1943 from aprescott/enqueue-and-perform-for-inline

Rely on perform_one for inline testing