gh-109723: Disable Py_BUILD_CORE in _testcapi (#109727)

Make sure that the internal C API is not tested by mistake by
_testcapi.

Undefine Py_BUILD_CORE_BUILTIN and Py_BUILD_CORE_MODULE macros in
Modules/_testcapi/parts.h: move code from _testcapimodule.c.

heaptype_relative.c and vectorcall_limited.c are using the limited C
API which is incompatible with the internal C API.

Move test_long_numbits() from _testcapi to _testinternalcapi since it
uses the internal C API "pycore_long.h".

Fix Modules/_testcapi/pyatomic.c: don't include Python.h directly,
just include _testcapi/parts.h.

Ajust "make check-c-globals" for these changes.