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.
But doesn't the other one ``postgresql-test-db`` change the global database, so presumably you'd want to change it back here?