Convert remaining JUnit3 tests to JUnit 4

Beside the obvious benefits of using the updated library, there are issues with test reliability as demonstrated by CONTINUUM-2736.

This may be a rather large amount of effort. While I converted all of the tests directly implementing TestCase already, most of the integration-style tests extend PlexusInSpring and AbstractContinuumTest, which indirectly extend TestCase and add considerable functionality.

As of today, these are using JUnit 3's TestCase:

• PlexusInSpringTestCase: 26
• AbstractContinuumTest: 23

PlexusInSpringTestCase is the superclass of AbstractContinuumTest, so after converting it, it might be easier to convert the remaining tests.