Add specs for Gitlab::Timeless

What does this MR do and why?

This MR adds missing specs for Gitlab::Timeless, simplify code by using yield model removing the unnecessary arity check, and replaces use of timeless with touch: false where possible.

The sole use of timeless is now in https://gitlab.com/gitlab-org/gitlab/-/blob/pl-gitlab-timeless/app/models/merge_request.rb#L249.

Spotted while working on #408971 (comment 1374933704).

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