Albert Strasheim mailed me the suggestion to run the Apache Commons Lang test suite (BTW, Albert was also the one who pointed me to the JRuby and MTJ test suites).
That also resulted in a number of bugs fixed/filed:
This list doesn't explain all failures, some are caused by the same issues as the ones mentioned and I think there are a few more GNU Classpath calendar/date/time etc. related bugs and a few tests are not quite "correct". For example, the ToStringBuilderTest.testReflectionHierarchyArrayList test assumes that the private member that contains the array of java.util.ArrayList is named elementData, but in GNU Classpath's implementation it is named data.