tools: assume good tests won't write to stderr

GObject silently catches exceptions and logs it to stderr without ever passing
on the exceptions. Avoid oversight by checking stderr in the tests.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

tools/ratbagctl.test.in

@@ -110,6 +110,8 @@ class TestRatbagCtl(unittest.TestCase):
     def launch_good_test(self, params):
         returncode, stdout, stderr = self.run_ratbagctl(params)
         self.assertEqual(returncode, 0, msg=stderr + stdout)
+        self.maxDiff = None
+        self.assertEqual(stderr, '')
         return stdout
 
     def launch_fail_test(self, params):