tests/unit/test_exit_if_null.c: Test through XMALLOC() instead of xaprintf()

tests/unit/test_exit_if_null.c: Test through XMALLOC() instead of xaprintf()

Both are indirect tests for exit_if_null(), but through XMALLOC() we
can test it more robustly, as we don't need to wrap vasprintf(3) to
make it fail.  It's trivial to make MALLOC(3) fail: pass a huge size.

The tests with xaprintf() were failing on Nix.  I suspect the compiler
was inlining aggressively, and as a result, the interposition of
vasprintf(3) in cmocka wasn't actually working.  The approach with
XMALLOC() seems to work on Nix, as we don't need to interpose malloc(3).
We still need to interpose exit(3), but for some reason that works fine.

Closes: <https://github.com/shadow-maint/shadow/issues/1382>
Reported-by: Silvan Mosberger <github@infinisil.com>
Tested-by: Silvan Mosberger <github@infinisil.com>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>