os/exec: remove flaky TestExtraFilesFDShuffle test

Update #5780

R=golang-dev, cshapiro, dave, bradfitz
CC=golang-dev
https://golang.org/cl/12869049

src/pkg/os/exec/exec_test.go

@@ -205,6 +205,7 @@ func closeUnexpectedFds(t *testing.T, m string) {
 }
 
 func TestExtraFilesFDShuffle(t *testing.T) {
+	t.Skip("flaky test; see http://golang.org/issue/5780")
 	switch runtime.GOOS {
 	case "darwin":
 		// TODO(cnicolaou): http://golang.org/issue/2603