Bug 21233: Unit tests

Bug 21233: Unit tests

This patch adds tests for the required password-related exceptions. The
tests verify the stringified version of the exceptions.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/Koha/Exceptions.t
=> FAIL: Exceptions not implemented, tests fail!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>