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

[Aleksey Shipilev]

On 1/23/19 12:27 PM, Roman Kennke wrote:
> 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?

OK!

-Aleksey