tests: add test for bogus NULL definition

tests: add test for bogus NULL definition

* tests/test-stdio.c: Ensure POSIX 2008 requirement on NULL.
* tests/test-stdlib.c: Likewise.
* tests/test-string.c: Likewise.
* tests/test-locale.c: Likewise.
* tests/test-unistd.c: Likewise.
* modules/stdio-tests (Depends-on): Add verify.
* modules/stdlib-tests (Depends-on): Likewise.
* modules/string-tests (Depends-on): Likewise.
* modules/locale-tests (Depends-on): Likewise.
* modules/unistd-tests (Depends-on): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>