[WIP] Setting up scaffolding for backend tests

[devforfu]

Establishes a setup for testing described in #84.

The purpose of this PR is to enable users for writing unit tests for the backend and, probably, discord-bot components of the Open-Assistant repo.

So far, it only has a trivial test that runs against a single API endpoint. (The app is instantiated in debug mode via the .env file.) But I decided to open a draft PR in order to keep the process transparent and communicate to other contributors. Also, to see if I'm on the right track with this issue.

cc @jack-michaud @andreaskoepf

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[devforfu]

I guess it was somehow automatically closed because of rebasing/merging the fork 🤔

I made one more attempt to restructure the dependencies a bit and try injecting them without a single "context" class including all possible repositories at once. Also, I had to move FrontendUserId into a separate file to avoid circular imports. Plus, I deleted one test as I am not sure how to deal with a nested @managed_tx_function.