Shenandoah: Faster and more parallel tests

Current hotspot_gc_shenandoah suite runs like this:

real 31m31.172s
user 287m50.380s
sys 5m28.277s

This amounts to 9.1x parallelism on 16-thread machine. Clearly, something big is on the critical path. Profiling shows the critical path is largely dominated by GCBasherWithShenandoah, which takes 26 minutes to run through all configurations. Other huge tests also run for a long time.

We can improve this as follows:
 *) Split @run into several @test groups. This would execute @test-s in parallel, reducing the impact on critical path;
 *) Remove some of the useless configurations from the tests;

Additionally, we can reformat @run sections to be more digestible.