DOC: add troubleshooting sec to README for unit tests

README_SETUP.md

@@ -82,6 +82,20 @@ with the then current version of the stored entities.
     * Alternatively, to run the tests in a containerized MariaDB instance, run `make test-docker`,
       followed by `make test-docker-stop`.
 
+### Troubleshooting
+
+#### MySQL has failing tests
+
+Our test suite is developed with mariadb. That is why some test, those which
+check constraints based on auto-generated names, fail because MySQL generates
+other names.
+
+Also some test fail with the message "Expected: NULL / Was: NULL" which is
+obviously what was expected.
+
+Please look carefully through the tests. If something more serious than that
+comes up, please report a bug.
+
 ## Documentation #
 
 Build documentation in `build/` with `make doc`.