net/http: don't test for goroutine leaks in short mode

Too annoying and flaky to debug for now. Later. This
tangent has taken enough time.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7863043

src/pkg/net/http/z_last_test.go

@@ -39,6 +39,9 @@ func interestingGoroutines() (gs []string) {
 // Verify the other tests didn't leave any goroutines running.
 // This is in a file named z_last_test.go so it sorts at the end.
 func TestGoroutinesRunning(t *testing.T) {
+	if testing.Short() {
+		t.Skip("not counting goroutines for leakage in -short mode")
+	}
 	gs := interestingGoroutines()
 
 	n := 0