Refactored xsum_sanity_check.c to use a table instead of direct
function calls. This is cleaner and has smaller code size.
Additionally, did some cleanup to reduce repetition a bit. There is
still a lot of DRY that could be applied to this file.
It now exists in xsum_sanity_check.c
Also add `XSUM_NO_TESTS` option, which instead prints "This version of xxhsum
is not verified." to stderr whenever XSUM_sanityCheck() is called.
