Pass assertion msg as a keyword argument in test_installation

It was originally made explicit like this, but it ended up becoming
position in my last few commits. This restores that clearer aspect
of how it was written before, while keeping all the other changes.

Author: EliahKagan

test/test_installation.py

@@ -63,7 +63,7 @@ def _check_result(self, result, failure_summary):
         self.assertEqual(
             0,
             result.returncode,
-            self._prepare_failure_message(result, failure_summary),
+            msg=self._prepare_failure_message(result, failure_summary),
         )
 
     @staticmethod