The UIState.ON_UPDATE notification should be enough to reset the state of the
panel every time the test is about to be executed. Wrong.
In response to this notification, gSync.onActivityStop() is called, which disables
the syncing animation immediately ONLY if it has been running for at least 1.5sec
(which is almost never happening in tests), otherwise we get a setTimeout with
the remaining time.
When the sync animation is running, we also do not react to clicks (disabled),
hence the test failing.
We fix this by stopping the animation ourselves manually at the end of the test,
so any subsequent test will start with a correct UI state.
MozReview-Commit-ID: CrEb82Tw5SO
d65914f3a7