tests/ndp: fix ndp_routeinfo_option testcase flakiness

Since we are sleeping for an indefinite period of time waiting
for the default route to appear, the expire times may be gone
past 1+ seconds, causing the Expire column to show <1800 or <600.

Fixes: f6bcc0925f0ea838da5183dc503f847e56d15cc8

Reviewed by: pouria
Approved by: lwhsu (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D56712