tests: Skip libarchive/selinux tests if in container without SELinux
tests: Skip libarchive/selinux tests if in container without SELinux

I'm doing builds and `make check` inside a Docker container, with
selinux on as a build-time option, but no policy in the container.
This currently aborts.  Let's not do that.

(This type of thing is why installed tests are a better model)

Closes: #546
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree