os/exec: disable TestExtraFilesFDShuffle

It's too hard to make portable just now.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/9057043

src/pkg/os/exec/exec_test.go

@@ -196,6 +196,8 @@ func basefds() uintptr {
 }
 
 func TestExtraFilesFDShuffle(t *testing.T) {
+	t.Skip("TODO: TestExtraFilesFDShuffle is too non-portable; skipping")
+
 	// syscall.StartProcess maps all the FDs passed to it in
 	// ProcAttr.Files (the concatenation of stdin,stdout,stderr and
 	// ExtraFiles) into consecutive FDs in the child, that is: