.gitlab-ci.yml: Use testsuite images for running tests

Instead of re-using the user-facing `buildstream/buildstream-fedora`
image for tests (that comes with BuildStream pre-installed), use
`buildstream/testsuite-fedora` image that is supposed to be used
exclusively for these tests.

Similarly, use the dedicated `buildstream/testsuite-debian` image
instead of `buildstream/buildstream-debian`.

Fixes #458.

For related discussion, see
buildstream-docker-images#8.