tests/mem_alloc: Eliminate alloc warnings. Define _BSD_SOURCE

We need to keep gcc from complaining about large calloc/malloc/realloc
sizes as that's what we're testing.

Define _BSD_SOURCE so that libraries sharing the glibc convention (like
newlib and picolibc) for controlling the API level visible to applications
will declare reallocarray(3), which is an OpenBSD extension to the C
library.

Signed-off-by: Keith Packard <keithp@keithp.com>