os: clean up after test

Noted in CL 31358 after submit.

Change-Id: I76ddad9b9e27dd6a03c1c4f49153213747fe0a61
Reviewed-on: https://go-review.googlesource.com/31365
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -836,6 +836,8 @@ func TestRenameToDirFailed(t *testing.T) {
 	Remove(to)
 	Mkdir(from, 0777)
 	Mkdir(to, 0777)
+	defer Remove(from)
+	defer Remove(to)
 
 	err := Rename(from, to)
 	switch err := err.(type) {