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.
diff --git a/Modules/Setup b/Modules/Setup
index 03aa0f1..e729ab8 100644
--- a/Modules/Setup
+++ b/Modules/Setup
@@ -173,6 +173,7 @@
 #_struct _struct.c	# binary structure packing/unpacking
 #_weakref _weakref.c	# basic weak reference support
 #_testcapi _testcapimodule.c    # Python C API test module
+#_testinternalcapi _testinternalcapi.c -I$(srcdir)/Include/internal -DPy_BUILD_CORE_MODULE  # Python internal C API test module
 #_random _randommodule.c	# Random number generator
 #_elementtree -I$(srcdir)/Modules/expat -DHAVE_EXPAT_CONFIG_H -DUSE_PYEXPAT_CAPI _elementtree.c	# elementtree accelerator
 #_pickle _pickle.c	# pickle accelerator