RFR: Rearrange Shenandoah tests into 3 tiers

[Roman Kennke]

Am 01.06.2018 um 20:51 schrieb Zhengyu Gu:
> Following upstream convention, rearrange Shenandoah tests into 3 tiers.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/test_groups/webrev.00/
> 
> Thanks,
> 
> -Zhengyu

It's good by me. Make sure Aleksey acks it, he probably needs to update
his test infra. Or, alternatively, keep hotspot_gc_shenandoah for
compat. :-)

Roman