fedora-langpacks: Skip self tests under non-fedora distros

The fedora-langpacks plugin disables itself if not on fedora. Skip the self tests in that situation as well so they don't fail. The reason this hasn't caused CI failures is that we have "--no-suite fedora-langpacks" in .gitlab-ci.yml so that it doesn't run under Debian. But it still seems worth skipping as people may run the tests manually, and we need to skip it in the downstream Endless CI.