Bug 1861 [QA Followup] - Fix Check_Userid and unit tests

Bug 1861 [QA Followup] - Fix Check_Userid and unit tests

Check_Userid assumes that a borrowernumber will always be passed in
and thus fails to to return 0 for an already used userid when creating
a new patron.

Unit tests must now also me modified to no longer assume it is possible
to create multiple patrons with the same userid.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit 7135364dd23442c38bed986b2540ab63cfd06ce1)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>