Bug 18579: Regression tests

Bug 18579: Regression tests

This patch makes sure the tests have the biblio.biblionumber mapping mocked
so we test the case where the mapping is to a control field instead of just
regular data fields (in the case of UNIMARC).

To test:
- Apply the patch
- Run:
  $ prove t/db_dependent/Koha/Filter/EmbedItemsAvailability.t
=> FAIL: Tests fail due to an attemp to access a subfield on a control field.

Sponsored-by: ByWater Solutions
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>