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>
...yep, just checked, they're not required.  I think I made this exception a long while ago for subtest/sub-subtest modules since documenting every class seemed like an unnecessary pain-in-the-butt.  These things work more like plugins anyway, no need to pervasive doc-strings.