test-rawmemchr: make more robust

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>