Fix nox session for slow CI tests

- when running tests marked with "r" we still must not run tests
  marked with "db" on the CI server

noxfile.py

@@ -207,7 +207,7 @@ def test(session):
     # test cases that require the slow installation of R and some packages.
     if session.env.get('_slow_ci_tests'):
         session.run(
-            'pytest', '--randomly-seed=4287', '-m', 'r', PYTEST_LOCATION,
+            'pytest', '--randomly-seed=4287', '-m', 'r and not db', PYTEST_LOCATION,
         )
 
         # In the "ci-tests-slow" session, we do not run any test tool