gh-151818: Add a test on CALL_LIST_APPEND error path

* Add _PyList_Capacity() internal function to _testinternalcapi.
* Add Modules/_testinternalcapi/list.c file.

Author: vstinner

Include/internal/pycore_list.h

@@ -77,6 +77,9 @@ union _PyStackRef;
 PyAPI_FUNC(PyObject *)_PyList_FromStackRefStealOnSuccess(const union _PyStackRef *src, Py_ssize_t n);
 PyAPI_FUNC(PyObject *)_PyList_AsTupleAndClear(PyListObject *v);
 
+// Export for '_testinternalcapi' shared extension
+PyAPI_FUNC(Py_ssize_t) _PyList_Capacity(PyListObject *self);
+
 #ifdef __cplusplus
 }
 #endif