Reduce Memory Usage of SingleThreadEventLoopTest

Motivation:

The SingleThreadEventLoopTest allocated up to half a gigabyte of memory per run,
causing up to 50 GC runs in ~20 seconds.

Modification:

Charlie Hunt identified TLAB allocations to be the root cause of this excessive
memory usage [1]. By reusing the Executor in every test we can reduce the
memory usage by >50%.

Result:

Lower Memory Usage and fewer Garbage Collector runs.
Helps to resolve GitHub issue #2841.

[1] https://twitter.com/charlesjhunt/status/505351389317722112
@normanmaurer `@Before` runs before a `@Test` method is executed. This would again lead to excessive thread creation and thus TLAB allocations. The point of this change is to use one `FJP` for all tests.

Also there is no need to shutdown a `ForkJoinPool` (according to its documentation).