use assertThat(a, not(b)) which a) works and b) provides better error me...

...ssages

I had some problems extending AggregateTest, and they seemed to be around JUnit's assertNotEquals. I have replaced it with assertThat, which is the better option anyway as it provides context when the test fails:
http://stackoverflow.com/questions/1096650/why-doesnt-junit-provide-assertnotequals-methods