batches/scheduler: remove flaky assertions

Created by: LawnGnome

As identified by @rvantonder in #22445, two specific assertions in ticker_test.go could result in test failures. Although ticker.stop() closes ticker.done, just draining that channel doesn't mean that ticker.C has been closed, as that happens asynchronously.

We could make ticker.stop() block until ticker.C is closed, but I don't think that's necessary or useful here: if the ticker's core goroutine is blocked waiting to send to another channel, this might take a while or — worse — cause a deadlock. ticker.stop() having lazy, asynchronous cleanup semantics seems reasonable to me.

(I did audit the other uses of stop(), but while there's a similar test construction in TestTickerZero, I don't believe it suffers from this problem, as there's no specific timing expected for ticker.C being closed.)