Bug 11338: add unit tests for DelItemCheck

Bug 11338: add unit tests for DelItemCheck

This patch adds unit tests for two parts of DelItemCheck: checking
if the item is on loan, and checking if it is waiting on the hold
shelf.

To test:

[1] Verify that prove -v t/db_dependent/Circulation/IsItemIssued.t
    is successful.
[2] Verify that prove -v t/db_dependent/Reserves.t is *not*
    successful -- as it turns out, there was a latent bug where items
    waiting on the hold shelf or in transit to fill a hold could still
    be deleted without any warning.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>