We're already using the return-value of cmp to print either PASS or
FAIL and in the case of failure, we're subsequently running and
showing the output of diff. So any warnings/errors from cmp itself are
not actually needed, and can be quite confusing.
As it turns out, 4 of our current tests are not valgrind clean,
(use after free errors or so), so this will be helpful for
investigating and fixing those.
