Fix graph/tags_unit_test.go on FreeBSD

- Directories should have u+x (0600 -> 0700)
- Don't chown to root.root by default because it requires root privilege

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
This is probably right but makes me very nervous because it's located in applylayer.  Why is this failing on freebsd?