Fix failing tests due to issues with wait_for_log_message (#7572)

- the test now waits for specific set of log messages rather than wait for
  timeout looking for just one message.
- we don't wanna sample the current length of the log after an action, due
  to a race, we need to start the search from the line number of the last
  message we where waiting for.
- when attempting to trigger a full sync, use multi-exec to avoid a race
  where the replica manages to re-connect before we completed the set of
  actions that should force a full sync.
- fix verify_log_message which was broken and unused