RFR (XS): 8232182: RedefineNestmateAttr/TestNestmateAttr.java failes due to ObjectCollectedException

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8232182
webrev: http://cr.openjdk.java.net/~dholmes/8232182/webrev/

Running this test with ZGC showed that a class we were still interested 
in could become unloaded. Simple fix is to store the Class reference in 
a static field.

Thanks,
David