port all Provider tests to JUnit4 semantics

Android recently switched from JUnit 3 to 4 for its base testing classes. It doesn't seem to support the old JUnit3 methods with gradle and AS. So all the tests need to be ported to JUnit4 to work again. I did all the tests except for the ones based on the FDroidProviderTest class, which I don't quite understand. I'm assigning this to @pserwylo since I think he wrote those tests. But anyone should feel free to beat him to it :)