Bug 17476: Add a way to bypass dt_from_string processing time for slow servers

Bug 17476: Add a way to bypass dt_from_string processing time for slow servers

We already change this value from 1 to 2 but apparently it's not enough
So let's switch it back to on and add a flag (env var SLOW_SERVER) and skip these tests
if set

Test plan:
  prove t/DateUtils.t
and
  SLOW_SERVER=1 prove t/DateUtils.t

should return green

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Mason James <mtj@kohaaloha.com>