Halt rather than poweroff the target system

At least on one test system, the BIOS option to turn the machine on
when power is restored distinguishes from a system-generated power
off to one not generated by the system, so after updating or running
tests, use 'systemctl halt' not 'systemctl poweroff'.
Increase the power-off time to 5 seconds

Before updating or testing, wait 5 seconds with the power off before
turning on the machine. There hasn't been any observed actual problems
with just a one second off time, but 5 seconds seems safer.