dbtest: skip updateDSNFromEnv if PGDATASOURCE specified
Created by: keegancsmith
This doesn't really make sense to do, since the user has already specified the exact string to use. We still keep the update code since that allows us to mutate the default test DSN.
This code could be better and I will follow-up with cleanups here. However, this fix makes it possible for me to run db tests locally again. Some recent refactors exposed issues in this code. In particular I use a unix socket for my postgres, and the interactions with that caused problems.