If a mtr test runs multiple servers and only some of them get
restarted on whatever reason with new command-line parameters,
then subsequent mtr test may fail, because no cleanup is performed.
Replication and Galera test suites are affected.

In the mtr script, there is a server_need_restart function
that decides whether we need to start a new mysqld process before
the new (next) test. If the mysqld parameters were changed in the
previous test - not necessarily the parameters of the primary mysqld
server, maybe even the secondary server parameters - this function
decides to start a new mysqld process. But since it does not remove
the old (changed) parameters, the new process starts with the
parameters changed by the *previous* test.

To correct this error, we must delete the modified process
parameters after checking that they have been changed during
the previous test.

This patch also simplifies and makes more stable the
galera_drop_database test, during debugging of which this
problem was detected.

https://jira.mariadb.org/browse/MDEV-17421