Fix intermittent spec failures in notify_spec.rb

What does this MR do?

Update a number of specs in notify_spec.rb to avoid intermittent failures

Are there points in the code the reviewer needs to double check?

Is have_referable_subject a good name?

Why was this MR needed?

Automatically generated names were being put, unescaped, into regular expressions. Where names included characters that had meaning in a regex, this would break the spec. Example: https://gitlab.com/gitlab-org/gitlab-ce/builds/12187485

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together