bpo-36635: Add _testinternalcapi module (GH-12841)

Add a new _testinternalcapi module to test the internal C API.

Move _Py_GetConfigsAsDict() function to the internal C API:
_testembed now uses _testinternalcapi to access the function.

Modules/_testinternalcapi.c

@@ -0,0 +1,45 @@
+/*
+ * C Extension module to test Python internal C APIs (Include/internal).
+ */
+
+#if !defined(Py_BUILD_CORE_BUILTIN) && !defined(Py_BUILD_CORE_MODULE)
+#  error "Py_BUILD_CORE_BUILTIN or Py_BUILD_CORE_MODULE must be defined"
+#endif
+
+#define PY_SSIZE_T_CLEAN
+
+#include "Python.h"
+#include "pycore_coreconfig.h"
+
+
+static PyObject *
+get_configs(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    return _Py_GetConfigsAsDict();
+}
+
+
+static PyMethodDef TestMethods[] = {
+    {"get_configs", get_configs, METH_NOARGS},
+    {NULL, NULL} /* sentinel */
+};
+
+
+static struct PyModuleDef _testcapimodule = {
+    PyModuleDef_HEAD_INIT,
+    "_testinternalcapi",
+    NULL,
+    -1,
+    TestMethods,
+    NULL,
+    NULL,
+    NULL,
+    NULL
+};
+
+
+PyMODINIT_FUNC
+PyInit__testinternalcapi(void)
+{
+    return PyModule_Create(&_testcapimodule);
+}