rate: use time.Since instead of time.Now().Sub

Change-Id: I140abf9d9283edb7259c9aa13ee69f3ca33181e6
Reviewed-on: https://go-review.googlesource.com/c/time/+/325609
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/rate/rate_test.go b/rate/rate_test.go
index 866347c..2491be7 100644
--- a/rate/rate_test.go
+++ b/rate/rate_test.go
@@ -401,7 +401,7 @@
 func runWait(t *testing.T, lim *Limiter, w wait) {
 	start := time.Now()
 	err := lim.WaitN(w.ctx, w.n)
-	delay := time.Now().Sub(start)
+	delay := time.Since(start)
 	if (w.nilErr && err != nil) || (!w.nilErr && err == nil) || w.delay != dFromDuration(delay) {
 		errString := "<nil>"
 		if !w.nilErr {