[Postgresql tests] Added enable_extension! to helper

cc/ @rafaelfranca, @sgrif , @senny

@sgrif, now the extensions after disabled after the tests like you asked.
We already own the connection when we call this method, this is why I think we should pass it. Right now it is coincidence that it is always `AR::Base.connection`. In this case I prefer the explicit call than the implicit implementation.