gh-95299: Rework test_cppext.py to not invoke setup.py directly (#103316)

gh-95299: Rework test_cppext.py to not invoke setup.py directly (#103316)

* gh-95299: Rework test_cppext.py to not invoke setup.py directly

* Add tests/cppextdata data to `TESTSUBDIRS`

* Revert "Add tests/cppextdata data to `TESTSUBDIRS`"

This reverts commit 635492e53954fb0fc2a2875c8961bde99266c48d.

* Revert "gh-95299: Rework test_cppext.py to not invoke setup.py directly"

This reverts commit 41c5a667b5de7070bbde5780f1c124f96863c91d.

* Build and install the extension in a temporary directory instead

* Pull in wheels for setuptools and wheel for testing extension builds