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

gh-128911: Add tests on the PyImport C API (#128915)

* 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.

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