AO3-2373 Adding tests to ensure related work emails are always sent

[ASalbenblatt]

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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  until they are reviewed and merged before creating new pull requests.

Issue

https://otwarchive.atlassian.net/browse/AO3-2373

Purpose

The issue appears to have been fixed at some point by unrelated code so this PR just adds tests to ensure that the issue stays fixed and is easily caught if it happens again.

Testing Instructions

1. Create a fic
2. Mark it as a translation or remix (I've only tested those) of an existing work of yours
3. Post it without previewing it
4. See if you get an email notification (you should)

Note: even though one comment says to "Add 'inspired by' to existing work" and then look for a notification, that is actually covered by issue AO3-1506 and so is not included in this PR

Credit

FlyingFalcon they/them