gh-106368: Argument clinic tests: improve error message when expect_success() fails (#107606)

Author: AlexWaygood

Lib/test/test_clinic.py

@@ -1680,7 +1680,8 @@ def run_clinic(self, *args):
 
     def expect_success(self, *args):
         out, err, code = self.run_clinic(*args)
-        self.assertEqual(code, 0, f"Unexpected failure: {args=}")
+        if code != 0:
+            self.fail("\n".join([f"Unexpected failure: {args=}", out, err]))
         self.assertEqual(err, "")
         return out