test-skeleton.c: xmalloc, xcalloc, xrealloc are potentially unused

test-skeleton.c: xmalloc, xcalloc, xrealloc are potentially unused

__attribute__ ((used)) means that the function has to be
emitted in assembly because it is referenced in ways the
compiler cannot detect (such as asm statements, or some
post-processing on the generated assembly).

The unused attribute needs to come first, otherwise it is
applied to the return type and not the function definition.