gh-95065: Argument Clinic: Add functional tests of deprecated positionals (#107768)

Move the "deprecated positinal" tests from clinic.test.c to
_testclinic.c. Mock PY_VERSION_HEX in order to prevent generated
compiler warnings/errors to trigger. Put clinic code for deprecated
positionals in Modules/clinic/_testclinic_depr_star.c.h for easy
inspection of the generated code.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>