Bug 15486: Unit tests

Bug 15486: Unit tests

This patch introduces unit tests for the new circulation rules option
that allows setting a max holds per day limit.

To test:
- Apply the patch
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/db_dependent/Holds.t
=> FAIL: CanItemBeReserved doesn't check the amount of holds per day
   and the introduced error code is not returned. OK is returned
   instead.

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>