Hello @atch,
You probably already had a look at them, but here are the main references when working with costum plugin for Java:
It is up-to-date with the current version (6.3) of the Java analyzer.
But on the prod, it is possible to get the FQN, but not on JUnitTest.
I would start by having a look at the POM, where you should add the dependencies you want to use in tests.
Then have a look at this test, and especially L39, where you should specify the test jar directory. Also, make sure that these dependencies are indeed present.
I hope it helps.
Quentin