Bugfix: do not set TESTMODULES environment variable if JDK is not modular

[Miloslav Zezulka]

This is a very small patch fixing the issue pointed out in $subject. I've
added checks for both compile and test JDK - is this an overkill, since
most of the time the implication goes COMPILEJDK.modular =>
TESTJDK.modular, or a necessary thing to have for foolproofness?

Please let me know what you think.

Thanks,
Mila
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