time: use an alternative method of yielding during Overflow timer test

Fixes #6874.

Use runtime.GC() as a stronger version of runtime.Gosched() which tends to bias the running goroutine in an otherwise idle system. This appears to reduce the worst case number of spins from 600 down to 30 on my 2 core system under high load.

LGTM=iant
R=golang-codereviews, lucio.dere, iant, dvyukov
CC=golang-codereviews
https://golang.org/cl/56540046

src/pkg/time/internal_test.go

@@ -78,7 +78,15 @@ func CheckRuntimeTimerOverflow() error {
 			if Now().After(stop) {
 				return errors.New("runtime timer stuck: overflow in addtimer")
 			}
-			runtime.Gosched()
+			// Issue 6874. This test previously called runtime.Gosched to try to yield
+			// to the goroutine servicing t, however the scheduler has a bias towards the
+			// previously running goroutine in an idle system. Combined with high load due
+			// to all CPUs busy running tests t's goroutine could be delayed beyond the
+			// timeout window.
+			//
+			// Calling runtime.GC() reduces the worst case lantency for scheduling t by 20x
+			// under the current Go 1.3 scheduler.
+			runtime.GC()
 		}
 	}
 }

src/pkg/time/sleep_test.go

@@ -398,6 +398,9 @@ func TestIssue5745(t *testing.T) {
 }
 
 func TestOverflowRuntimeTimer(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode, see issue 6874")
+	}
 	if err := CheckRuntimeTimerOverflow(); err != nil {
 		t.Fatalf(err.Error())
 	}