tests: Wrap calloc by just returning NULL if we're called too early

Since glibc dlsym() calls calloc, we get a call to our calloc wrapper as
we try to look up the real calloc implementation.  dlsym() will fall back
to a static buffer in case calloc returns NULL, so that's what we'll do.

This is all highly glibc dependent, of course, but the entire malloc
weak symbol wrapper mechanism is, so there's no loss of generality here.