AUM OTA manual: copy hint with time settings for all tests

Use timedatectl to workaround NTP issues on some networks

With battery backed devices on networks with no properly reachable NTP servers
the boards would fail to automatically get the right time, so testers set it up
manually with `date`.

However, the hardware clock need to be configured as well for the time
configuration to survive a reboot, needed by the Apertis update manager OTA
test.

Without this, the device will fail to establish any TLS connection after the
reboot since every certificate will look like they are out of their validity
period.

This is only a workaround until we ensure NTP servers are reachable on the
testers' network, see https://phabricator.apertis.org/T6115



Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>