Use local files for testing ui.interactive_image

[falkoschindler]

This PR tries to fix our flaky pytests for ui.interactive_image. It seems like most failures came from fetching dummy images from a remote server, which could even block requests with a rate limit. Therefore I replaced those images with locally served copies.

I also overloaded the screen.wait_for method to allow waiting for a condition. This is handy when waiting for events in test_loaded_event.

Furthermore I noticed a minor bug in screen.should_not_contain:

self.selenium.implicitly_wait(wait)
with pytest.raises(AssertionError):
    self.find(text)
self.selenium.implicitly_wait(self.IMPLICIT_WAIT)

In case pytest.raises itself raised, the implicit wait hasn't been set back to IMPLICIT_WAIT. I fixed it with a try-finally block.

[rodja]

Great!