Bug 18382: (QA follow-up) Replace ->ymd by ->datetime

Bug 18382: (QA follow-up) Replace ->ymd by ->datetime

The first patch makes Hold.t fail on:
not ok 8 - Hold is suspended with a date, truncation takes place automatically
   Failed test 'Hold is suspended with a date, truncation takes place automatically'
   at Hold.t line 94.
          got: '2018-03-22'
     expected: '2018-03-22T00:00:00'

We could remove the midnight time there, but we also could replace ymd by
datetime. The cpan doc tells us:
    Same as $dt->ymd('-') . 'T' . $dt->hms(':')

Test plan:
With this patch, run Hold.t again. It should pass now.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>