RFR(XS): 8224529: [TESTBUG] JFR TestShenandoahHeapRegion* tests fail on build w/o Shenandoah

[Erik Gahlin]

Looks good.

Erik

> Please review this small change that fixes at-requires statement for 
> JFR tests that require Shenandoah GC.
> Change also adds vm.gc.Shenandoah to JDK's TEST.ROOT
>
>     JBS: https://bugs.openjdk.java.net/browse/JDK-8224529
>     Webrev: http://cr.openjdk.java.net/~mseledtsov/8224529.00/
>     Testing:
>         1. Ran the affected tests on a build w/o Shenandoah: tests 
> skipped as expected
>         2. Runnin entire jdk_jfr on a test cluster - in progress
>
>
> Thank you,
> Misha
>