Need a way to augment JTREG_LAUNCHER_OPTIONS from command-line

A common problem when diagnosing test failures is that the log has its middle cut out due to hitting jtreg's default log limit. The way to fix that when running jtreg directly is to pass -J-Djavatest.maxOutputSize=<new value>.

However when jtreg is being run via the test makefiles (including when submitting via mach5 and needing to pass the flag) there's no way to pass this value.

Discussion excerpt:

David Holmes [3:27 PM] How do you pass `-J-Djavatest.max
OutputSize=NNN` to jtreg via mach5? From the command-line I can just specify it as written. But I edited Runtests.gmk to add it to JTREG_BASIC_OPTIONS and that got:
```jib > Error: Invalid option: -J-Djavatest.maxOutputSize=10000000```

Magnus Ihse Bursie [11:03 PM] Ok. I expect you need to add it as -D to the launcher. Look in RunTest for JTREG_LAUNCHER_OPTIONS.
David Holmes [11:04 PM] Right. It's being passed as a jtreg arg rather than an arg to the jvm
Magnus Ihse Bursie [11:06 PM] Unfortunately there is no way right now to add it to the command line. But you can hack RunTest for now to solve your pressing issue, and we can fix it properly. Either if this option should always be passed to the launcher, or if we should add a way to pass options to the launcher. (Or both!)
I got to leave now. Please file a bug if you want me or a Erik to fix something.