db.postgresql: configurable test db setting

This pr is so you can add something like `<postgresql-db> "name" >>username "template1" >>database \ postgresql-db set-global` in your .factor-rc and have the postgres tests run against that db instead of the hardcoded one. It's similar to the `imap-settings` global.
Nah, the `database` slot in `postgresql-db` should already contain the `"template1" string. Or whatever the name is of your template database if you have changed it.