RFR: Integrate Shenandoah tests to upstream test groups

[Zhengyu Gu]

Integrated Shenandoah tier1 tests to upstream tier1 test groups, and 
prevent tier1 tests to step into gc/shenandoah directory to pick up 
other tests.

Also, moved shenandoah test groups into more proper section.

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

Test:

  Eyeball tier1_gc and tier1 tests.

Thanks,

-Zhengyu