Allow setting an app_builder attribute on a TestCase.

This is to avoid the annoying requirement of overriding __init__
in every test case. For example, this now works:

    class MyTestCase(CycloneTestCase):
        app_builder = MyApplication

        def test_get(self):
            self.client.get("/")

In order to avoid hacky messing with *args, the app_builder can now
only be a kwarg. The relevant test has been updated to the new
behaviour.