0.21.11: Allow to use socket and IPv6 in database connection

Make DB-connection compatible with wordpress: allow to use sockets and IPv6.
It's mostly for new users, who uses non-standard database connection settings.
If you are existing user, you dont' need this update.

* Improve database-related testing aspects
* Stabilize auto-tests
* Upgrade testing environment
* Parse wordpress's DB_HOST constant in same way as it was introduced in WP 4.9
with respect to socket and IPv6.