`WAREHOUSE_DATABASE_URL` could be parsed to issue the `createdb` and `dropdb` commands with the given credentials. i'm not super concerned one way or the other, @dstufft or other developers may have additional input.

the major advantage of the ephemeral postgres instance was---in my opinion---the fact that it doesn't require someone to concern themselves with any of these things for running tests. a developers **only** concern with regard to postgres, was to have it installed.