Bug 36480: Add Koha::Library->desks

Bug 36480: Add Koha::Library->desks

We add an accessor for the related desks. Tests are added.

To test:
1. Apply this patch
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Koha/Library.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 7f7f486d94e52eb50d070ccead61c94928aeff28)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>