tests: rewrite hwclock test

Major changes:
 - don't use deprecated ntpdate but sntp client
 - don't update sys time from ntp, just compare offsets
 - now hwclock is the only command which needs root permissions
 - resolve ntp server to one fixed IP to get comparable offsets
   always from the same remote machine
 - don't enter the systohc/hctosys loop if not even --show works
 - verbose but not too ugly debug output

Note, maybe later the resolve_host function could be moved to functions.sh
because other tests (e.g. last) should check whether dns is available.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>