blackbox tests: add timestamps for subunit tests

blackbox tests: add timestamps for subunit tests

There is the icky thing with sed because some kinds of `date` don't
have sub-second resolution, which we really want.

Another way to do it would be:

   python -c "import datetime; print datetime.datetime.utcnow().strftime('time: %Y-%m-%d %H:%M:%S.%fZ')"

which should be universal, but is a little slower.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>