fix tests

[royneary]

1. Make tests compile with caqti >= 2.0.0
2. Fix an sqlite error message (I'm using sqlite 3.50.4)
3. Use postgresql's peer authentication method. I couldn't figure out how to specify a postgres password, so I removed localhost:5432 from the postgresql URIs. Now, if a postgres user and database with the same name as my unix user exists, authentication works without password. I create the postgres user like this:

   CREATE USER christian CREATEDB;
   CREATE DATABASE christian OWNER christian;
   

4. Allow running dune test without the --profile=test flag by removing all the (enabled_if (= %{profile} test)) conditions. Please let me know if these conditions are still important.