Local source/target socket connections only work for compiled-in default

[mbanck-cd]

For (regression) tests, it would be useful to be able to connect to a local temporary postgres instance without having to setup users and passwords.

This works if the unix sockets are in the compiled-in default location, by setting host: "". However, for the source: and target: sections (it does work fine for the migrator: connection), setting a unix socket directory explicitly makes the parser trip over like this:

2025-09-30 08:21:36,005: [ERROR] Error preparing session settings: connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: FATAL:  database "var/run/postgresql:5433/postgres" does not exist

So that means it is not possible to use a different socket directory. Debian's pg_virtualenv (which we would like to use for some basic regression tests) creates a temporary instance in /tmp and places its socket in /tmp, so using pg_virtualenv without passwords is currently not possiblt