[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 76c26eaca4147ba7e3e8d7379c5a828f0b512a46)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>