`chronyc waitsync 60 0.01` will wait up to about 10 minutes (60 times 10
seconds) for chronyd to synchronise to a source and the remaining
correction to be less than 10 milliseconds [1].
Also flush handlers to restart chronyd before synchronising the clock.
Force system clock synchronisation in CI jobs. This allows us to run
test_ntp_clock_synchronized on Ubuntu again.
[1] https://chrony-project.org/doc/4.6.1/chronyc.html
Change-Id: I73e7f2a3258e5427ec16b01e7482093d7bfb33f8
8dd05487d4