Changing the internal default to use JT Harness 5.0 will cause classfile version problems when testing much-older (pre 1.7) versions of JDK.
Therefore, http://hg.openjdk.java.net/code-tools/jtreg/rev/c2bfb30bd8c6 needs to be reverted.

But at the same time, JTHarness 5.0 no longer requires JavaHelp in any form, so the makefiles can be updated so that JavaHelp is an optional dependency, only required when using JTHarness 4.6.