test: generate per-user email

[Turbo87]

When generating a test user, derive that user's email address from the username.

This makes asserting the correct recipient emails are used when inspecting emails in tests.

@domodwyer I extracted that from one of your branches, since it seems like a useful change on its own and is also used/needed for #2705 :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.16%. Comparing base (02d0087) to head (1642204).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9322   +/-   ##
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  Coverage   89.16%   89.16%           
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  Files         282      282           
  Lines       28904    28904           
=======================================
  Hits        25772    25772           
  Misses       3132     3132           

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[domodwyer]

Glad it's useful!

There's also b397f42 which helps validate which email is sent to which user - you can see it in action here: e97e93b - maybe it helps? 🙏

FYI I do plan on polishing up that branch and submitting it soon - I figured it was better to get the previous refactor in first!

[domodwyer]

Whoops

Suggested change

	/// (`<username>@example.com``) and return a mock user session.
	/// (`<username>@example.com`) and return a mock user session.