Enhanced test coverage for strncmp, wcsncmp

Add string/test-strncmp-nonarray and
wcsmbs/test-wcsncmp-nonarray.

This is the test that uncovered bug 31934.  Test run time
is more than one minute on a fairly current system, so turn
these into xtests that do not run automatically.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit 54252394)