This bug tracks the mass marking of tests to make on-device testers green. The intention is to sort through these tests in the future.
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Is this not just a dupe of Bug 173275?
It's not. Gardening means getting the test suite to green by mechanically skipping failing tests, and bug 173275 tracks investigating what makes results different from simulator. Making on-device a thing will almost certainly require simultaneous progress on both, but they are still separate tasks.