hg: shenandoah/jdk: Move Shenandoah stress tests to tier3

[zgu at redhat.com]

Changeset: 333e3e31243d
Author:    zgu
Date:      2018-07-27 13:18 -0400
URL:       http://hg.openjdk.java.net/shenandoah/jdk/rev/333e3e31243d

Move Shenandoah stress tests to tier3

! test/hotspot/jtreg/TEST.groups