tstime/rate: deflake TestLongRunningQPS

This test is highly dependent on the accuracy of OS timers.
Reduce the number of failures by decreasing the required
accuracy from 0.999 to 0.995.
Also, switch from repeated time.Sleep to using a time.Ticker
for improved accuracy.

Updates #2727

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

tstime/rate/rate_test.go

@@ -182,15 +182,18 @@ func TestLongRunningQPS(t *testing.T) {
 		wg.Done()
 	}
 
+	// This will still offer ~500 requests per second,
+	// but won't consume outrageous amount of CPU.
 	start := time.Now()
 	end := start.Add(5 * time.Second)
-	for time.Now().Before(end) {
+	ticker := time.NewTicker(2 * time.Millisecond)
+	defer ticker.Stop()
+	for now := range ticker.C {
+		if now.After(end) {
+			break
+		}
 		wg.Add(1)
 		go f()
-
-		// This will still offer ~500 requests per second, but won't consume
-		// outrageous amount of CPU.
-		time.Sleep(2 * time.Millisecond)
 	}
 	wg.Wait()
 	elapsed := time.Since(start)
@@ -201,7 +204,7 @@ func TestLongRunningQPS(t *testing.T) {
 		t.Errorf("numOK = %d, want %d (ideal %f)", numOK, want, ideal)
 	}
 	// We should get very close to the number of requests allowed.
-	if want := int32(0.999 * ideal); numOK < want {
+	if want := int32(0.995 * ideal); numOK < want {
 		t.Errorf("numOK = %d, want %d (ideal %f)", numOK, want, ideal)
 	}
 }