test:Fix sucker_punch flaky test

[marcotc]

Fixes https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/1691/workflows/ebaf9777-0229-49e2-9172-438841cea5d4/jobs/81378:

  1) sucker_punch instrumentation delayed job should instrument enqueuing for a delayed job
     Failure/Error: is_expected.to be true
     
       expected true
            got false
     # ./spec/ddtrace/contrib/sucker_punch/patcher_spec.rb:118:in `block in <main>'
     # ./spec/ddtrace/contrib/sucker_punch/patcher_spec.rb:23:in `block in <main>'

This error happens because SuckerPunch::Job#perform_in schedules the job to be performed and then checks if the job status is :pending right before returning. If the job finishes too quickly, it might have already been moved into a :fulfilled, thus making #perform_in return false.

This PR does not verify the return value of #perform_in anymore, as it is unimportant to our tests, but maintains the verification that the job has finished and that the correct spans have been generated.

[ericmustin]

👍