RFR(T) 8221688: Quarantine Shenandoah string dedup tests

[Roman Kennke]

Yes, please! Also, please cherry-pick it into shenandoah/jdk straight away.

Thanks, Roman

> I would like to quarantine Shenandoah string deduplication tests until 
> JDK-8220671 and JDK-8221102 are fixed.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8221688
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8221688/webrev.00/
> 
> Thanks,
> 
> -Zhengyu