[54] automatically test endpoints

[Mateusz-Dobrzynski]

I figured out a way to perform endpoint tests using Rust.

• #[tokio::test] allows running tests in Tokio runtime, making it possible to run a server and perform requests.
• I populated the tests/ with some integration tests. They can be run with cargo run along with unit tests or separately with cargo run --tests, the only prerequisite being that the database must be up (ofc).
• Unit tests can be run separately with cargo run --lib, so I changed the Github Actions workflow accordingly.
• Contrary to unit tests, which are run in parallel, integration tests are run in serial, as enforced by the #[serial] macro. This is because the database is a critical resource. This prolongs execution time, but ensures tests' isolation and stability. A single module (in this case tests contained within a single file) can be run with cargo test --test [module_name].