Improve tests that set system properties.
Tests in the clogr-logback and clogr-logback-provider repositories set a system property to be used in tests. Changing a system property might have side effects in other tests. The tests need to make sure the system property is reset back to what it was before the tests.
See How Stable are Your Unit Tests?. We'll probably want to create a separate globalmentor-junit repository for the type of JUnit utility mentioned there.