So here is a little something to circumvent these restrictions:
1) Create a new Java-Project where your tests will be located
2) right-click on it and go into "Build Path" -> "Configure Build Path..."

3) select the "Project"-Tab and click on "Add" proceed by selecting the Lejos project you want to test.

Now you can proceed to write tests using your favorite Frameworks as if you were writing them in the lejos project itself.