os: do not leave /go_os_test/dir after test

Change-Id: Idb8e211bf33d2713735f9cdc868a3458ce799c97
Reviewed-on: https://go-review.googlesource.com/6913Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/path_test.go

@@ -213,7 +213,7 @@ func TestMkdirAllAtSlash(t *testing.T) {
 		}
 	}
 	RemoveAll("/_go_os_test")
-	const dir = "/go_os_test/dir"
+	const dir = "/_go_os_test/dir"
 	err := MkdirAll(dir, 0777)
 	if err != nil {
 		pathErr, ok := err.(*PathError)