tests: Remove interactive tests

The tests rely on the ability to run Clutter as stand-alone app, which hasn't been possible for quite a while now. Event handling was broken long before that, plus tests that need a human to run them and evaluate the result aren't that useful to begin with.

It would be neat to test our CSS rendering by setting up a stage, capturing it and comparing it to a reference image. But writing some code that exercises backgrounds, borders etc. is hardly the obstacle to make that happen, and not worth carrying around dead code.