ctdb-tests: Clean up and rename simple fetch_ring test

ctdb-tests: Clean up and rename simple fetch_ring test

* Rename to clarify purpose of test

* Simplify test info to avoid unnecessary bit-rot

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

* Simplify sanity checking pattern due to less data because of
  --interactive/-i option

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>