ping tests: Fix test on i386 platforms

Actually use INT_MAX (0x7fffffff), not UINT_MAX (0xffffffff) to avoid
overflowing time_t and missing the test.

Reported by: Jenkins
Reviewed by: markj
Fixes: 5b8af90fe332 ("ping: Add ATF-Python tests")
Pull Request: https://github.com/freebsd/freebsd-src/pull/874
Differential Revision: https://reviews.freebsd.org/D42268