os: allow EROFS in TestMkdirAllAtSlash

On some systems (e.g. ChromeOS), / is mounted read-only.
This results in error code syscall.EROFS, which I guess
is just as valid as syscall.EACCES for this test.

Change-Id: I9188d5437a1b5ac1daa9c68b95b8dcb447666ca3
Reviewed-on: https://go-review.googlesource.com/3676Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/path_test.go

@@ -211,7 +211,7 @@ func TestMkdirAllAtSlash(t *testing.T) {
 	if err != nil {
 		pathErr, ok := err.(*PathError)
 		// common for users not to be able to write to /
-		if ok && pathErr.Err == syscall.EACCES {
+		if ok && (pathErr.Err == syscall.EACCES || pathErr.Err == syscall.EROFS) {
 			return
 		}
 		t.Fatalf(`MkdirAll "/_go_os_test/dir": %v`, err)