time: make TestReset more reliable

Fixes #4690

R=golang-dev, alex.brainman, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7181052

src/pkg/time/sleep_test.go

@@ -247,26 +247,49 @@ func TestSleepZeroDeadlock(t *testing.T) {
 	<-c
 }
 
-func TestReset(t *testing.T) {
-	t0 := NewTimer(100 * Millisecond)
-	Sleep(50 * Millisecond)
-	if t0.Reset(150*Millisecond) != true {
-		t.Fatalf("resetting unfired timer returned false")
+func testReset(d Duration) error {
+	t0 := NewTimer(2 * d)
+	Sleep(d)
+	if t0.Reset(3*d) != true {
+		return errors.New("resetting unfired timer returned false")
 	}
-	Sleep(100 * Millisecond)
+	Sleep(2 * d)
 	select {
 	case <-t0.C:
-		t.Fatalf("timer fired early")
+		return errors.New("timer fired early")
 	default:
 	}
-	Sleep(100 * Millisecond)
+	Sleep(2 * d)
 	select {
 	case <-t0.C:
 	default:
-		t.Fatalf("reset timer did not fire")
+		return errors.New("reset timer did not fire")
 	}
 
 	if t0.Reset(50*Millisecond) != false {
-		t.Fatalf("resetting expired timer returned true")
+		return errors.New("resetting expired timer returned true")
+	}
+	return nil
+}
+
+func TestReset(t *testing.T) {
+	// We try to run this test with increasingly larger multiples
+	// until one works so slow, loaded hardware isn't as flaky,
+	// but without slowing down fast machines unnecessarily.
+	const unit = 25 * Millisecond
+	tries := []Duration{
+		1 * unit,
+		3 * unit,
+		7 * unit,
+		15 * unit,
+	}
+	var err error
+	for _, d := range tries {
+		err = testReset(d)
+		if err == nil {
+			t.Logf("passed using duration %v", d)
+			return
+		}
 	}
+	t.Error(err)
 }