RFR: Shenandoah test groups should not include each other

[Roman Kennke]

According to Leonid's review, tierN+1 should not include tierN. This
also seems consistent with other tierX groups. So now, if you want to
run all tests, you need to specify all 3 tierX_gc_shenandoah groups, or
we decide to add a hotspot_gc_shenandoah_all group or such.

http://cr.openjdk.java.net/~rkennke/shenandoah-test-groups/webrev.00/

Testing: tier1,2,3_shenandoah_gc, eyeballing executed tests

Ok?

Roman