RFR: Prepare tests for upstreaming

[Zhengyu Gu]

1) Added guards for Shenandoah tests in shared directories
2) Removed duplicated Critical Native tests, shared them among 
Shenandoah and Epsilon.


Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/test_reshuffle/webrev.00/


Test:
   Eyeball tier1_gc_1 and tier3_gc_shenandoah, make sure tests are ran.

Thanks,

-Zhengyu