test-rawmemchr: make more robust * modules/rawmemchr-tests (Files): Add zerosize-ptr.h, mmap-anon.m4. (Depends-on, configure.ac): Add needed prerequisites to use it. * modules/memchr-tests (Files, Depends-on, configure.ac): Likewise, to avoid implicit reliance on memchr module prereqs. * tests/test-memchr.c (main): Ensure proper masking. * tests/test-rawmemchr.c (main): Likewise. Detect oversized reads. Signed-off-by: Eric Blake <eblake@redhat.com>