[ticket/11295] Drop tables rather than database for postgres in test suite.

Doing so allows:

1. User running the tests no longer needs create database privilege.
2. Test database may be located in a non-default tablespace and generally
have site-specific options applied to it.

PHPBB3-11295
Merge remote-tracking branch 'p/ticket/11295' into develop-olympus

* p/ticket/11295:
  [ticket/11295] Drop tables rather than database for postgres in test suite.