Improve camel-quarkus-master leader/follower test

If the clustering wouldn't work, than it could happen that the test would write `follower` because both would write to that file all the time and it would be just about when the `follower` is catched by the `readLeaderFile`. 

I'm suggesting improvement in a way that both routes would write to different files. And we check the `follower` file is empty and after we kill the leader, checking the file is not empty.