The previous implementation would print
./tools/check-gitlab-ci.sh: line 22: ci-fairy: command not found
also, it's not nice to execute the program, just to find out whether
we have it.
This will be used on our gitlab-ci tests. We do have a separate,
dedicated test on gitlab-ci for checking that "gitlab-ci.yml" is
correct.
We may intentionally want to violate that condition, but then our
`make check` should not fail.
Add a way to skip that test.