mutablelimiter: Fix flakey test
Created by: keegancsmith
If a machine has high load there is a chance that TestLimiter would fail. "ctx3" has a timeout of 250ms, which is low enough that the call to SetLimit won't cancel contexts in time. So we increase the timeout drastically to reduce the chance of the flakey failure.
Additionally we use a better pattern for waiting on "ctx1" to be cancelled.