Improvement in unit test resource usage

Motivation:
Some unit tests which use multiple NioEventLoopGroup objects have failed due to OutOfMemoryExceptions.
This is casued by two reasons:
1. Issue #2837
2. The tests are generally not re-using Executor object when they could which leads to alot of thread/memory allocations

Modifications:
-HTTP/2 unit tests which are subject to this problem will be corrected to re-use Executors
-Handler unit tests which used HTTP/2 tests as templates will also be updated

Result:
-No more OOM exception
-More efficient unit tests
That will tell the `ForkJoinPool` to maintain `Short.MAX_VALUE` many threads (~32000?) ;-).