tests: Clear messages between test cases

Makes watching the UI (or screenshots) more useful as we don't have any
leftovers from previous tests.

tests/end2end/fixtures/quteprocess.py

@@ -549,6 +549,7 @@ class QuteProc(testprocess.Process):
     def before_test(self):
         """Clear settings before every test."""
         super().before_test()
+        self.send_cmd(':clear-messages')
         self.send_cmd(':config-clear')
         self._init_settings()
         self.clear_data()