pythongh-106368: Argument clinic tests: improve failure message when …

…tests in `ClinicExternalTests` fail (python#107364)

Lib/test/test_clinic.py

@@ -1386,7 +1386,7 @@ def _do_test(self, *args, expect_success=True):
         ) as proc:
             proc.wait()
             if expect_success and proc.returncode:
-                self.fail("".join(proc.stderr))
+                self.fail("".join([*proc.stdout, *proc.stderr]))
             stdout = proc.stdout.read()
             stderr = proc.stderr.read()
             # Clinic never writes to stderr.