RFR (S) 8193622: JFR test TestUnloadingEventClass.java times out intermittently

[coleen.phillimore at oracle.com]

Summary: Previous change was leaving scratch classes on CLD::_klasses 
list which are reported to tracing

Tested with assert in tracing code and failed test, and mach4 tier1-5.

open webrev at http://cr.openjdk.java.net/~coleenp/8193622.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8193622

This change is relative to jdk/hs repository but will be moved to 
jdk/jdk10 once the jdk/hs snapshot is integrated.

Thanks,
Coleen