Bug 13933: Fix t/DateUtils.t without DB

Bug 13933: Fix t/DateUtils.t without DB

If the mysql server is not running, the DateUtils.t tests failed.
This is caused by

commit dd9f456ab7c2f0e66173a6feb7df2a38604c82cd
    Bug 13601: Add tests to highlight the problems with
    DateTime::Format::DateParse

The pref dateformat and TimeFormat were retrieved before the mock.
Before bug 13601, the complete preference method was mocked, that's why
the error did not appear.

Test plan:
  sudo service mysql stop
  prove t/DateUtils.t
should be happy

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>