Fix: suppress warning about unknown pytest mark.

Author: bjones1

pytest.ini

@@ -0,0 +1,7 @@
+# ****************************
+# |docname| - Configure pytest
+# ****************************
+[pytest]
+# See `Registering marks <https://docs.pytest.org/en/stable/how-to/mark.html>`_; this mark is defined in `selenium_module_fixture`.
+markers =
+    exit_status_success: Determine if the runestone build exit code is checked by ``selenium_module_fixture``.