Move tests to a module, include in subclasses, style linting

This fixes an error where the test runner would try and run
XMLMiniEngineTest like a normal test class, except it's abstract. Now,
to circumvent this, we don't include any of the actual tests in
XMLMiniEngineTest; they are wrapped in a module that is included in
subclass when they inherit from XMLMiniEngineTest. Pretty neat, huh?