hg: shenandoah/jdk: Fix TestUnifiedLoggingSwitchStress test failure

[zgu at redhat.com]

Changeset: 85c98984ed89
Author:    zgu
Date:      2018-06-01 14:24 -0400
URL:       http://hg.openjdk.java.net/shenandoah/jdk/rev/85c98984ed89

Fix TestUnifiedLoggingSwitchStress test failure

! src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.cpp
! test/hotspot/jtreg/TEST.groups
! test/hotspot/jtreg/gc/logging/TestUnifiedLoggingSwitchStress.java