[11u] RFR (XS) 8256806: Shenandoah: optimize shenandoah/jni/TestPinnedGarbage.java test

[Aleksey Shipilev]

Original fix:
   https://bugs.openjdk.java.net/browse/JDK-8256806
   https://github.com/openjdk/jdk/commit/86f36027

It does not apply cleanly to 11u, because test got swept in global "randomness" refactoring. I redid 
it to use the same ThreadLocalRandom as it did in current 11u.

11u webrev:
   https://cr.openjdk.java.net/~shade/8256806/webrev.11u.01/

Testing: affected test on Linux x86_64

-- 
Thanks,
-Aleksey