RFR: JDK11-only: Skip CriticalNative* tests when Shenandoah is not present

[Roman Kennke]

I noticed that when building without Shenandoah, the 2 CriticalNative* 
tests are failing because they are not filtered on Shenandoah. This 
change fixes that.

Testing: hotspot_gc_shenandoah with and without Shenandoah

http://cr.openjdk.java.net/~rkennke/sh11-fix-critical-tests/webrev.00/

OK?

Roman