This prevents clients on volatile networks from running heuristics repeatedly,
which has two consequences: performance hit due to the DNS lookups performed by
heuristics, as well as flooding telemetry since we record an event each time.
With this patch, when a heuristics run is first triggered, we run it as usual
but start a timer in the background. Any runs triggered before the timer fires
are coalesced into a count. When the timer fires, if the count exceeded a limit,
we restart the timer and also the count. If the count was within the limit, we
run heuristics just once if it was triggered at least once while throttling.
To make this testable deterministically, I had to make a few subtle changes,
e.g. to not await calls to runHeuristics.
I also took the opportunity to replace BrowserTestUtils.waitForCondition with
the more canonical copy that lives in TestUtils, and to put the confirmationNS
pref into the prefs object so it doesn't need to be explicitly cleaned up.
Differential Revision: https://phabricator.services.mozilla.com/D110358
db1e6bc9f8