Make sure debug_flags is available for unit tests

tests/helpers/fixtures.py

@@ -441,6 +441,7 @@ def fake_save_manager():
 def fake_args(request):
     ns = types.SimpleNamespace()
     ns.backend = 'webengine' if request.config.webengine else 'webkit'
+    ns.debug_flags = []
     objreg.register('args', ns)
     yield ns
     objreg.delete('args')