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

[Roman Kennke]

> 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

Yes, looks fine! Thanks!

Roman