Bug 27209: (follow-up) Add ->is_pickup_location_valid

Bug 27209: (follow-up) Add ->is_pickup_location_valid

This patch simply refactors the pickup location check into a method that
can be called on its own. Tests are added, and the tests for
->set_pickup_location should pass unmodified.

To test:
1. Apply the first two patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Hold.t
=> SUCCESS: tests pass
3. Apply this refactoring patch
4. Notice the tests are similar, but check for boolean output
5. Repeat 2
=> SUCCESS: New tests pass, set_pickup_location() behavior unchanged.
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>