strncmp: Add a testcase for page boundary [BZ #25933]

strncmp: Add a testcase for page boundary [BZ #25933]

Add a strncmp testcase to cover cases where one of strings ends on the
page boundary with the maximum string length less than the number bytes
of each AVX2 loop iteration and different offsets from page boundary.

The updated string/test-strncmp fails on Intel Core i7-8559U without

ommit 1c6432316bc434a72108d7b0c7cfbfdde64c3124
Author: Sunil K Pandey <skpgkp1@gmail.com>
Date:   Fri Jun 12 08:57:16 2020 -0700

    Fix avx2 strncmp offset compare condition check [BZ #25933]