Fix an issue with logical replication test and simplify the test

[osaxma]

• Fixed a string concatenation issue that made the test fail before.
• updated docker_process in dev_dependencies
• Simplified the test by adding configurations directly to usePostgresContainer.

---

On another note, I tried to make the logical replication test work on the CI in my fork but with no success. I could not find a way to pass image arguments to the container in the current setup of dart.yaml. I think a custom container would be needed but maybe that can be done another time.

For reference, mounting a volume with the current setup also wasn't straight forward. Here's the workaround for future reference:

    services:
      postgres:
        .....
        volumes:
         # mounting the volume
          - ${{ github.workspace }}/test/custom_config/postgresql.conf:/etc/postgresql/postgresql.conf
        .....

    steps:
      # This step must be before `checkout`
      # This solves an issue that prevents mounting the volume due to `checkout` cleanup job
      # For more details, see: https://github.com/actions/checkout/issues/211#issuecomment-611986243
      - name: Change owner for workspace to prevent volume mounting issue in checkout action
        run: sudo chown -R $USER:$USER ${{ github.workspace }}