tests: Stabilize test_qute_settings_persistence

The :jseval command triggers an XHR in JS, and we'll need to wait for that to
actually finish (async) before the setting is really set.

See https://github.com/qutebrowser/qutebrowser/issues/5390#issuecomment-628773737

tests/end2end/test_invocations.py

@@ -381,6 +381,8 @@ def test_qute_settings_persistence(short_tmpdir, request, quteproc_new):
     quteproc_new.send_cmd(':jseval --world main '
                           'cset("search.ignore_case", "always")')
     quteproc_new.wait_for(message='No output or error')
+    quteproc_new.wait_for(category='config',
+                          message='Config option changed: *')
 
     assert quteproc_new.get_setting('search.ignore_case') == 'always'