[devtools] make PostgresTestDataBaseHandler start its own postgresql Don't depend on postgresql already running on the system, or on a static config. To use this, set the configcls attribute of your CubicWebTC-derived test class to PostgresApptestConfiguration. Caveats: - this won't work with several tests running concurrently from the same directory, since each would start its own cluster with a shared data directory and stomp on each other's toes - you need initdb/pg_ctl in $PATH, e.g /usr/lib/postgresql/$version/bin on Debian/Ubuntu systems. Closes #3489631