Most of the mtr tests in the galera_3nodes suite fail
for a variety of reasons with a variety of errors.
Some tests simply need to add the missing "connection"
lines to the result files, but many of them fail due
to substantial errors that require reworking test files.

This patch adds the missing "connection" lines to
the result files and fixes several substantial flaws
in the galera_3nodes suite tests and in the mtr framework
service files, adapting the tests from galera_3nodes
for the current version of MariaDB.

https://jira.mariadb.org/browse/MDEV-18426