Isolate unit tests

Running the unit tests for Wagon produced the attached Surefire report. Symptomatic is that the first test fails while all following raise errors. The Surefire report shows that this is due to

BindException: Address already in use: JVM_Bind

and indeed, I couldn't spot a call to tearDownWagonTestingFixtures() in case the test method was completed abnormally, so the previously started Jetty server is still hanging around when the next test tries to run.

What about moving the tearDown*() methods into the usual tearDown() method invoked by JUnit? The alternative of using try/finally blocks everywhere might be cumbersome. Additionally, tear down should be fail-safe, e.g. a call like server.stop() would need to be guarded against server being null.