RFR (XS) 8236917: TestInstanceKlassSize.java fails with "The size computed by SA for java.lang.Object does not match"

[coleen . phillimore]

Summary: Use getTestJavaOpts() instead of getVmOptions() because of 
mach5 configuration settings.

Tested with hs-tier3 with no failures anymore.
open webrev at http://cr.openjdk.java.net/~coleenp/2020/8236917.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8236917

Thanks,
Coleen