tst: Add test for futimens

This patch provides test for futimens. It uses wrapper, which
reads access and modification time to compare them with ones
written by futimens.

Moreover, access and modification times beyond the Y2038 threshold
date (i.e. 32 bit time_t overflow) are also checked.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>