Well I tried adding Paul's malloc.c as suggested in his bug report.
It did not work since it gave an error about redefining malloc() . This was not surprising since there was no malloc.c where he suggested putting it so it was bound to be a double declaration. Maybe his suggestion was aimed at an earlier version.
Anyway, once I removed it again it must have force a rebuild of something since I now got the error I was expecting from the incorrect naming of the free() replacement in stdlib.h
Correcting both names to be good_free() now works and a test shows same memory before and after your stest() call.
0000 0820 0824 : used/free/large
0000 0820 0824 : used/free/large
It would be good to have full instructions on how to force the necessary rebuild for your replacement free() to take effect. It's something to do with restarting the IDE but that's not enough on its own, I'd done that several times already.
Thanks for providing the code.