Some of the changes in bug 1626083 result in a more deterministic flow that uncovers a race
and an incorrect underlying assumption in the test.
The underlying assumption is that clearing mode on shutdown is disabled in tests, which is false - it's enabled.
The race is that we assume that the mode is already 0 and try to ensure it doesn't change, whereas it actually
starts empty (undefined) and changes to 0, because of the incorrect underlying assumption.
This mistake only exists in browser_cleanFlow.js, the other tests are correctly waiting for the final TRR mode
without trying to ensure no change.
Differential Revision: https://phabricator.services.mozilla.com/D114592