gh-106368: Argument clinic tests: improve failure message when tests in `ClinicExtern...

diff --git a/Lib/test/test_clinic.py b/Lib/test/test_clinic.py
index d21c7d84c88d2dad18102589b07648aad716d834..2f74ee29b204dbf41f5e4dc2e539198e699a1457 100644 (file)
--- a/Lib/test/test_clinic.py
+++ b/Lib/test/test_clinic.py
@@ -1386,7 +1386,7 @@ class ClinicExternalTest(TestCase):
         ) 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.