Bug 18460: Fix undefined itemtype warning in Serials.t

Bug 18460: Fix undefined itemtype warning in Serials.t

This patch makes the test create an itemtype, and use it for the created item so there's no warning.

To test:
- Run:
  $ prove t/db_dependent/Serials.t
=> FAIL: item-level_itypes set but no itemtype set... warning raised
- Apply the patch
- Run:
  $ prove t/db_dependent/Serials.t
=> SUCCESS: Tests pass and no warning is raised
- Sign off :-D

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>