README.unittest container edits mainly mariadb/mysql (#7014)

README.unittest container edits mainly mariadb/mysql (#7014)

Simplify it a bit:

* test_schema2 isn't needed in MySQL/MariaDB test.
* Only permissions on test_schema are needed.
* User without qualification defaults to user@'%'.
* Use MARIADB_* names with MariaDB container that has
  been there most of this year in releases.
* We don't need to specify a database on connection
  when creating users/databases.
* On all containers - docker will always pull image
  if not available. Also postgres image didn't match
  container initialization.
* Remove container versions, just use latest