Use MARIADB_AUTO_UPGRADE=1 to recreate healthcheck users cnf file missings

Factor our create_healthcheck_users.

Make sure that when recreating users, if they already exist, we just password
reset these and ensure the .my-healtcheck.cnf file is there for usage. We don't
want to clobber any existing grants if we happen not to have MARIADB_HEALTHCHECK_GRANTS
set.

Because creating users needs to move past --skip-grant-tables with FLUSH PRIVEGES,
and mariadb-upgrade also issue FLUSH PRIVILEGES, then unfortunately is yet another
restart.

Adjust test case to ensure there is no .cnf file, and create it on
restore.