RFR: Test @requires should check what GC is enabled at runtime

[Roman Kennke]

> http://cr.openjdk.java.net/~shade/shenandoah/requires-runtime/webrev.01/
> 
> This rewires our @requires handling again! @requires is messy, and new filtering code is based on
> understanding that:
>  a) "vm.gc.Shenandoah" says "Shenandoah is available in builds, and can be selected"
>  b) "vm.gc == null" says "...and we are running with default GC"
>  c) "vm.gc = Shenandoah" says "...or user specified -XX:+UseShenandoahGC in jtreg opts"
> 
> ...which makes all tests that set -XX:+UseShenandoahGC gain the filter like:
>  @requires vm.gc.Shenandoah & (vm.gc == "null" | vm.gc == "Shenandoah")
> 
> Testing: hotspot_gc_shenandoah with:
>   - -shenandoahgc build
>   - +shenandoahgc build
>   - +shenandoahgc build, TEST_VM_OPTS="-XX:+UseShenandoahGC"
>   - +shenandoahgc build, TEST_VM_OPTS="-XX:+UseParallelGC"

I needed to think again. So what you proposed selects the test if:
- Shenandoah is actually available (vm.gc.Shenandoah)
*and* one of:
  - harness did not force a GC
  - harness forced ShenandoahGC

which seems good. Go!

Thanks!
Roman