lastcomm: update amd64 and i386 tests for additional timestamp resolution

Fixes: 6f4ce7e89ba4 ("lastcomm: improve timestamp display")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f079ef5b6a3c6d8800724286675199f79b553ccb)
(cherry picked from commit 407e2d7cf1b6762c9768eaff60500b8d007cd81c)