HTTP/2 Unit Tests EventLoopGroup cleanup

Motivation:
The HTTP/2 unit tests are suffering from OOME on the master branch.
These unit tests allocating a large number of threads (~706 peak live on a quad core machine) which may
be related to this memory pressure.

Modifications:
Each EventLoopGroup shutdown operation will have a `sync()` call.

Result:
Lower peek live thread count and less associated memory pressure.