debian/tests/timedated: Check enabling/disabling NTP

Assert that `timedatectl set-ntp` correctly controls the service, sets
the `org.freedesktop.timedate1 NTP` property, and sends the right
`PropertiesChanged` signal.

This reproduces <https://github.com/systemd/systemd/issues/11944> and
also the earlier <https://github.com/systemd/systemd/issues/9672>.