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

[mikhailo.seledtsov at oracle.com]

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