gh-94751: Install, import and run the test C++ extension (MVP) (GH-94754) (#94780)

This is a quick-and-dirty way to run the C++ tests.
It can definitely be improved in the future, but it should fail when things go wrong.

- Run test functions on import (yes, this can definitely be improved)
- Fudge setuptools metadata (name & version) to make the extension installable
- Install and import the extension in test_cppext
(cherry picked from commit ec5db539b9df99c8b96149768dc2e8598dce2afa)

Co-authored-by: Petr Viktorin <encukou@gmail.com>