A few of the tests connected to the paint signal but never
disconnected it. Most of these handlers had a call to g_main_quit in
them which meant that it could sometimes cause subsequent tests to
exit after the first frame is painted. Most of the tests don't
validate any of the results until after a couple of frames have been
rendered so this ended up skipping out the test entirely.
To workaround this the test setup function now disconnects all
handlers for the paint signal on the default stage before the test is
run.
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