Tests: improve how processes are killed

[LeoNero]

This PR does three things:

• renames setup_clients to launch_farcasterd_instances
• removes cleanup_processes and adds a struct FarcasterdProcess with a impl Drop; this has the advantage that processes will die even if tests fail, which was not happening before
• improve the logic that cleans up processes; it had some problems that processes spawned by farcasterd were not being killed

Run the tests (force them to fail too), and in all cases check if any farcaster-related process is running in your machine.

[h4sh3d]

In #707 we took another approach to kill processes. I like the implementation of Drop to cleanup process, it's very rust way of thinking. Do you want to give it a shot and try to port your work based on the state of main?

[h4sh3d]

Superseded by #707