More robustness needed when setting up JavaParser's ProjectRoot

I have encountered some fragility with the interface extension mechanism around
working out the root directory for JavaParser's
ProjectRoot. I
will soon upload a test case that shows a scenario where the choice of JDK for
the schema compilation makes the difference between succeeding, and failing
with "Unable to construct a common project root - giving up.". IMHO
sorting this issue out needs more than simply getting a test case to work,
rather some thought is needed about what is really required.