tests: don't hardcode expected perm bits in chown test

"chown other" asserted mkdir produces "drwxr-xr-x", but that depends
on the container's umask, which isn't what the test is verifying.
Capture the actual perm bits after mkdir and assert chown leaves them
unchanged, regardless of what the ambient umask happens to produce.

Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>