ISPN-4631 Make sure test servers are shut down before/after tests

https://issues.jboss.org/browse/ISPN-4631
Right, I forgot that JUnit actually creates a new instance for each test method. There's still the question, though, is Arquillian going to run `stopJoiningNodes()` if a previous test failed? Because if it doesn't, you're going to get a failure in another, unrelated test.