Cirrus-CI: Check for final "Uptime" message

The last thing printed by the kernel after shutdown is Uptime: <time>.
In the CI boot test script add a check for this, to ensure that shutdown
is successful. A test branch of mine panicked during shutdown, but
still reported success in Cirrus-CI.

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48823