subtests.docker_cli: Add new test run_volumes_selinux

This test uses `docker run --volume ::Z/z` and checks it sets the
selinux context properly.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
`R0903` means too few public methods, I copied this from `kill` test and this version probably doesn't even need it (It has 2 public methods). I'll remove it on Monday.