RFR: Move Shenandoah stress tests to tier3

[Zhengyu Gu]

Currently, our stress tests are in tier2. The six stress tests take more 
to run than the rest of 82 tests, which makes little difference, in term 
of testing time, between tier2 and tier3.

By moving stress tests to tier3, tier2 takes about 20 minutes to run on 
my local machine (vs. about 1 hour), which is more reasonable testing 
time for trivial changes.


Wberev: http://cr.openjdk.java.net/~zgu/shenandoah/sh-test-tiers/webrev.00/

Thanks,

-Zhengyu