non-deterministic testUserSubscriberUsingRequestAsync

fix non-deterministic failures of BackpressureTests.testUserSubscriberUsingRequestAsync

I was able to replicate the occasional failure by putting it in a tight loop. With these changes it no longer fails.
Merge pull request #1498 from benjchristensen/testUserSubscriberUsingRequestAsync

non-deterministic testUserSubscriberUsingRequestAsync