hg: shenandoah/jdk11: Skip CriticalNative* tests when Shenandoah is not present

[roman at kennke.org]

Changeset: 54a6ec6eb3e5
Author:    rkennke
Date:      2019-01-23 12:22 +0100
URL:       http://hg.openjdk.java.net/shenandoah/jdk11/rev/54a6ec6eb3e5

Skip CriticalNative* tests when Shenandoah is not present

! test/hotspot/jtreg/gc/shenandoah/jni/CriticalNativeArgs.java
! test/hotspot/jtreg/gc/shenandoah/jni/CriticalNativeStress.java