gh-93649: Split heaptype tests from _testcapimodule.c (GH-95386)

This removes the unused negative_dictoffset function:
the type this function would create is available as
    _testcapi.HeapCTypeWithNegativeDict

Author: encukou

Modules/Setup.stdlib.in

@@ -169,7 +169,7 @@
 @MODULE__XXTESTFUZZ_TRUE@_xxtestfuzz _xxtestfuzz/_xxtestfuzz.c _xxtestfuzz/fuzzer.c
 @MODULE__TESTBUFFER_TRUE@_testbuffer _testbuffer.c
 @MODULE__TESTINTERNALCAPI_TRUE@_testinternalcapi _testinternalcapi.c
-@MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c
+@MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/heaptype.c
 
 # Some testing modules MUST be built as shared libraries.
 *shared*