[3.12] gh-128911: Add tests on the PyImport C API (GH-128915) (GH-128960) (#128989)

* Add Modules/_testlimitedcapi/import.c
* Add Lib/test/test_capi/test_import.py
* Remove _testcapi.check_pyimport_addmodule(): tests already covered
  by newly added tests.
(cherry picked from commit 34ded1a1a10204635cad27830fcbee2f8547e8ed)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
(cherry picked from commit d95ba9fa1110534b7247fa2ff12b90e930c93256)