testing: use time.Since in highPrecisionTime

time.Since has optimizations for measuring monotonic time.

For #31160.

Change-Id: I0529b9f69b9f008e3414b8e386b6faa64af4a008
Reviewed-on: https://go-review.googlesource.com/c/go/+/582135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/src/testing/testing_other.go b/src/testing/testing_other.go
index 586f61b..f91e3b4 100644
--- a/src/testing/testing_other.go
+++ b/src/testing/testing_other.go
@@ -27,5 +27,5 @@
 
 // highPrecisionTimeSince returns duration since b.
 func highPrecisionTimeSince(b highPrecisionTime) time.Duration {
-	return time.Now().Sub(b.now)
+	return time.Since(b.now)
 }