I've also tried building on a vanilla-ish 10.8 VM, both without and with installing the /usr/include headers, and I still don't see the failure you're seeing.  If you look at Modules/posixmodule.c, you'll see there's a fair amount of Apple-specifc or -caused hackery to deal with past and current getgroups() anomalies.  Perhaps you could step through it on your system and see if it's obvious what's happening.

Also, looking back through previous issues with getgroups, in Issue29562 Ronald noted:

"Note that the result of getgroups(2) is fixed on login, while "id -G" reflects the current state of the user database on macOS. Could this explain this failure? That is, have you tried logging out and in again before running the test suite?"