time: more deflaking asynctimer tests

For #66322.

Change-Id: I1d83c7a3cacd2ab012039d954270a7c87bbdf5ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/584195
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Russ Cox <[email protected]>

Author: rsc

src/time/tick_test.go

@@ -440,7 +440,7 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 		Sleep(sched)
 		tim.Reset(10000 * Second)
 		if isTicker {
-			assertTick()
+			drain()
 		}
 		noTick()
 
@@ -466,8 +466,7 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 	Sleep(sched)
 	tim.Reset(10000 * Second)
 	if !synctimerchan && isTicker {
-		assertLen()
-		assertTick()
+		drain()
 	}
 	noTick()