tests/conftest.py: Get the template params from the actual template name

instead of the entire string which can contain other data, such as the
bus name.

Also adds an assert to ensure template is actually of type string which
makes mypy happy.

tests/conftest.py

@@ -403,9 +403,10 @@ def templates(
         dbusmock.BusType.SYSTEM: {},
         dbusmock.BusType.SESSION: {},
     }
-    for template, params in required_templates.items():
+    for template_data, params in required_templates.items():
+        template, bus_name = (template_data.split(":") + [None])[:2]
+        assert template
         params = template_params.get(template, params)
-        template, bus_name = (template.split(":") + [None])[:2]
         _start_template(busses, template, bus_name, params)
     yield
     _terminate_servers(busses)