Improvement in unit test resource usage

Motivation:
Some unit tests which use multiple NioEventLoopGroup objects have failed due to OutOfMemoryExceptions.
This is caused by at least two reasons:
1. Issue #2837
2. The tests are generally not re-using EventLoopGroups when they could which leads to a lot of thread/memory allocations

Modifications:
-HTTP/2 unit tests which are subject to this problem will be corrected to re-use EventLoopGroups
-Handler unit tests which used HTTP/2 tests as templates will also be updated

Result:
-Less chance of OOM exception because of more efficient use of resources
-Faster & more efficient unit tests