RFR (S) 8242271: Shenandoah: add test to verify GC mode unlock

[Roman Kennke]

Yup.

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8242271
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8242271/webrev.01/
> 
> Pretty much a copy of gc/shenandoah/options/TestHeuristicsUnlock.java.
> 
> Testing: new test in fastdebug/release modes
>