string: Add a testcase for wcsncmp with SIZE_MAX [BZ #28755]

Verify that wcsncmp (L("abc"), L("abd"), SIZE_MAX) == 0.  The new test
fails without

commit ddf0992c
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Sun Jan 9 16:02:21 2022 -0600

    x86: Fix __wcsncmp_avx2 in strcmp-avx2.S [BZ# 28755]

and

commit 7e08db33
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Sun Jan 9 16:02:28 2022 -0600

    x86: Fix __wcsncmp_evex in strcmp-evex.S [BZ# 28755]

This is for BZ #28755.

Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>

(cherry picked from commit aa5a7200)