docs: improve developer tutorial on how to run tests, incl. coverage

.coveragerc

@@ -1,2 +1,2 @@
 [run]
-omit = */tests/*, */scripts/*
+omit = */tests/*, */scripts/*, **/*.jinja, **/*.html, **/*.txt

documentation/dev/introduction.rst

@@ -148,25 +148,40 @@ You can run automated tests with:
 
 .. code-block:: bash
 
-   $ make test
+    $ make test
 
 
-which behind the curtains installs dependencies and calls pytest.
+which behind the curtains installs dependencies and calls ``pytest``.
 
-A coverage report can be created like this:
+However, a test database (postgres) is needed to run these tests. If you have postgres, here is the short version on how to add the test database:
 
 .. code-block:: bash
 
-   $ pytest --cov=flexmeasures --cov-config .coveragerc
+    $ make clean-db db_name=flexmeasures_test db_user=flexmeasures_test
+    $ # the password for the db user is "flexmeasures_test"
+
+.. note:: The section :ref:`host-data` has more details on using postgres for FlexMeasures.
+
+Alternatively, if you don't feel like installing postgres for the time being, here is a docker command to provide a test database:
+
+.. code-block:: bash
 
+    $ docker run --rm --name flexmeasures-test-db -e POSTGRES_PASSWORD=flexmeasures_test -e POSTGRES_DB=flexmeasures_test -e POSTGRES_USER=flexmeasures_test -p 5432:5432 -v ./ci/load-psql-extensions.sql:/docker-entrypoint-initdb.d/load-psql-extensions.sql -d postgres:latest
 
-You can add --cov-report=html after which a htmlcov/index.html is generated.
+.. warning:: This assumes that the port 5432 is not being used on your machine (for instance by an existing postgres database service).
+
+If you want the tests to create a coverage report (printed on the terminal), you can run the ``pytest`` command like this:
+
+.. code-block:: bash
+
+   $ pytest --cov=flexmeasures --cov-config .coveragerc
 
-It's also possible to use:
+You can add `--cov-report=html`, after which a file called `htmlcov/index.html` is generated.
+Or, after a test run with coverage turned on as shown above, you can still generate it in another form:
 
 .. code-block:: bash
 
-   $ python setup.py test
+    $ python3 -m coverage [html|lcov|json]