os/exec: dump lsof on failure

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5504063

src/pkg/os/exec/exec_test.go

@@ -262,6 +262,7 @@ func TestHelperProcess(*testing.T) {
 			f, err := os.Open(os.Args[0])
 			if err != nil {
 				fmt.Printf("error opening file with expected fd %d: %v", wantfd, err)
+				fmt.Println(Command("lsof", "-p", fmt.Sprint(os.Getpid())).CombinedOutput())
 				os.Exit(1)
 			}
 			if got := f.Fd(); got != wantfd {