Added a sleep for onscreen rendered tests

This should help generalize what @durack1 did in #1341 (adding a sleep to handle the race condition with resized windows). The root of the issue is #1148; I have some tests that resize the window (to make sure everything is well-behaved), and because of #1148 they have to be rendered onscreen. This makes them run slowly, which looks like it adds a race condition– see intermittent failures of the resize manager and background color tests. This makes it so the base class checks if the window is set to render offscreen; if it isn't, it will sleep.:MERGED: