Postgres authentication fails: log refers to "line 90" in pg_hba.conf that shouldn't exist
User jjanes' comment helped me find the real hba-file. It's actually here:

/etc/postgresql/12/db_test/pg_hba.conf

(and not in the /var/lib/postgresql/12/... Don't remember why I had expected it there. As I'm new to all this, the branches of a linux directory tree are misterious to me.)