In-depth testing

[Lonami]

We should aim for a good test coverage. For example, things like the transports deserve a test (abridged doesn't even work right now!), but ideally we would review the code or apply some coverage tools to get a better idea of what deserves more attention.

[Lonami]

In particular, the sender should make sure that all the buffers are reused and cleared correctly, what happens when two messages arrive at once or only one and partial, etc. Context: https://t.me/gramme_rs/5226. Probably adding a mock server that replies twice.

[Lonami]

The CI should also test with all features enabled.