Add test that the viewer isn't stuck in infinite update loop

[aedm]

We should add a few sanity checks (probably in the integration test suite) that the viewer eventually renders without getting stuck in an infinite repaint loop.

[emilk]

I think that should be part of kittest. harness.run() should run until there are no more request_repaint, and if that time doesn’t come reasonably quickly, that indicates a forever-repaint bug, and should cause a test failure imho.