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 54252394c2)
