string: Fix tester build with fortify enable with gcc < 12

When building with fortify enabled, GCC < 12 issues a warning on the
fortify strncat wrapper might overflow the destination buffer (the
failure is tied to -Werror).

Checked on ppc64 and x86_64.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>