RFR: 8214484: ZGC: Exclude SA tests ClhsdbJhisto and TestHeapDumpFor*

[Per Liden]

There's no support for SA heap walking when using ZGC. Hence the tests 
for this should not execute when ZGC is enabled.

Bug: https://bugs.openjdk.java.net/browse/JDK-8214484
Webrev: http://cr.openjdk.java.net/~pliden/8214484/webrev.0
/Per