test: Automation of the unit testing environment

[avdb13]

Currently testing feels like moping a floor under a running tap, mainly because of the following problems:

1. As we're not exposing the homeserver configuration to the end-user (for now), we need a way to automate the generation of it
2. We have to run just backend separately before testing and restart in case changes are made to the aforementioned file
3. There's no clear way of separating errors caused by the problems mentioned above yet from actual endpoint errors

[avdb13]

Relevant article: https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html

Unit tests for request/responses could be implemented more flexibly with a macro that tries out all possbible variants of a given type. Let's rely on API tests only for now until all required Matrix endpoints for the alpha release are set in stone.