date: Fix tests by initializing timespec structure

The tests related to nanosecond support were failing on amd64 due to
uninitialized timespec structure.

Fixes: eeb04a736cb9 date: Add support for nanoseconds
Reviewed by: markj
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D44975

(cherry picked from commit 7b390cb636899582243e4daa8edbdb30d686d9fb)