RFR(T) 8221688: Quarantine Shenandoah string dedup tests

[Zhengyu Gu]

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