[3.12] gh-106368: Argument clinic tests: improve failure message when tests in ClinicExternalTests fail (GH-107364) (#107365)

gh-106368: Argument clinic tests: improve failure message when tests in `ClinicExternalTests` fail (GH-107364)
(cherry picked from commit 76c26ea)

Co-authored-by: Alex Waygood <[email protected]>

Author: miss-islington

Lib/test/test_clinic.py

@@ -1360,7 +1360,7 @@ 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.