gitlab-ci: add a FreeBSD test job

This uses the new FreeBSD supported added in
https://gitlab.freedesktop.org/freedesktop/ci-templates/-/merge_requests/114


After the latest series of commits, we should be able to compile and
successfully run all tests, so adding this CI job will prevent any future
FreeBSD regressions.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>