Support Jihu's fast_spec_helper

What does this MR do and why?

Resolve #3340 (closed)

Related Upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/127574

Just like spec_helper.rb, we should also have Jihu's own fast_spec_helper.rb.

This way we can easily use skip_specs.yml to skip "fast specs".

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