string: Fix tester with fortify enabled

If fortify is enabled, the truncated output warning is issued by
the wrapper itself:

In function ‘strncpy’,
    inlined from ‘test_strncpy’ at tester.c:505:10:
../string/bits/string_fortified.h:95:10: error: ‘__builtin_strncpy’
destination unchanged after copying no bytes from a string of length 3
[-Werror=stringop-truncation]
   95 |   return __builtin___strncpy_chk (__dest, __src, __len,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   96 |                                   __glibc_objsize (__dest));
      |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../include/bits/string_fortified.h:1,
                 from ../string/string.h:548,
                 from ../include/string.h:60,
                 from tester.c:33,
                 from inl-tester.c:6:
In function ‘strncpy’,
    inlined from ‘test_strncpy’ at tester.c:505:10:

Checked on x86_64-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>