test: raft leader election

This patch introduces two tests:

1. couple.test_failover reproduces heatbeat timeout on a follower that leads to a new election.

2. threesome.test_leader_dispuption simulates disconnected follower. It shouldn't disrupt the leader in this case.

Leader death is already tested in threesome.test_log_rollback.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/25