selftest: consistently produce high-res UTC time

selftest: consistently produce high-res UTC time

Currently some subunit reporters throughout the codebase provide low-res time,
meaning timestamps jump back and forth in the subunit file. Also, some subunit
reporters produce UTC timestamps while others produce local time. UTC was chosen
as the standard for this commit since all of the timestamps end with a Z (= Zulu
= UTC).

Signed-off-by: Jamie McClymont <jamiemcclymont@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Mar 22 13:26:44 CET 2018 on sn-devel-144