ctdb-tests: Clean up and rename simple message_ring test

ctdb-tests: Clean up and rename simple message_ring test

* Rename to clarify purpose of test

* Simplify test info to avoid unnecessary bit-rot

* Have message_ring print PNN for clearer output and update patterns
  in test script to suit.

* Drop quantitative percentage check since this is hard to predict
  when under extreme load.  To compensate, tighten up expectations for
  the total number of messages in each direction: at least 10 messages
  are required over 10 seconds.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12109

Pair-programmed-with: Amitay Isaacs <amitay@gmail.com>
Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>