Revert "testing: delete unused stopAlarm()"

This reverts commit ed8c62b7fb.

Turns out it was needed in later commits.

Change-Id: I07a7bc2429976d8a5a89f915a11625c118b85500
Reviewed-on: https://go-review.googlesource.com/36113
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/testing/testing.go

@@ -1027,6 +1027,13 @@ func startAlarm() {
 	}
 }
 
+// stopAlarm turns off the alarm.
+func stopAlarm() {
+	if *timeout > 0 {
+		timer.Stop()
+	}
+}
+
 func parseCpuList() {
 	for _, val := range strings.Split(*cpuListStr, ",") {
 		val = strings.TrimSpace(val)