tests: Temporarily allow tests to pass with warnings

Florian Müllner requested to merge fmuellner/gnome-shell:unblock-tests into main

One of the tests now triggers a warning on shutdown almost reliably.

A fix is being worked on on the mutter side; until that lands, let's not block all shell merge requests or waste CI resource by re-running the test until we hit the race condition that avoids the warning.

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