require haml lint spec helpers

The haml lint repo has useful spec helpers. https://github.com/sds/haml-lint/tree/master/spec/support

They are so useful I have copy and pasted them to our repos in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11457.

I have opened a GH issue https://github.com/sds/haml-lint/issues/302 and a PR to move these files to published lib/ dir will be accepted.