ISPN-4029 Reworked NotifyingFutureTest

Hi

Please take a look at reworked NotificationFutureTest. These are the steps I performed

1. Removed Thread.sleep() since it can't be trusted (especially when testing on CI server with many slaves)
2. It turned out that all testDone tests were acting exactly the same without Thread.sleep. So I combined them.
3. I reworked the test a bit and added custom assertions.

Best regards
Sebastian
That's good point.. I'll refactor individual `get`. Thanks for pointing that out!