[11u] RFR (S) 8251944: Add Shenandoah test config to compiler/gcbarriers/UnsafeIntrinsicsTest.java

[Aleksey Shipilev]

Original test improvement:
   https://bugs.openjdk.java.net/browse/JDK-8251944
   https://git.openjdk.java.net/jdk/commit/db6f3930

It does not apply cleanly to 11u, because test still has @requires !graal and ZGC is not yet 
production-ready and needs UnlockExperimentalVMOptions. Otherwise the patch is the same:
   https://cr.openjdk.java.net/~shade/8251944/webrev.11u.01/

Testing: affected test

-- 
Thanks,
-Aleksey