docker/test: add a pytest based test for the git loading stack

For this, we extract the docker-compose setup in a dedicated fixture.
This is needed otherwise one cannot run all tests at once (both scheduler_host
and deposit_host conflict on the docker-compose part).

Also no need for most of the fixtures to be session scoped.

In docker/scheduler/entrypoint.sh, run setup_pgsql before setup_pip;
this hack is needed to prevent some race conditions in tests where it
sometimes happens that the ~/.pg_service file generated by the setup_pgsql
bash function is not visible in first calls to scheduler_host.check_output()
testinfra helper function.

Running setup_pip after setup_pgsql ensures(?) this does not happen.