Add a unit test that ensures that deleting a device purges the associated refresh token

[anoadragon453]

Plugs a hole in our unit tests for refresh tokens.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
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• Code style is correct (run the linters)

[reivilibre]

Thanks!

Ideally we'd have FKs for this but this is certainly better than nothing