Bug 27858: Unit tests

Bug 27858: Unit tests

This patch introduces unit tests for the new exception to be thrown.
It generates a new extended attribute type, keeps the randomly generated
'code' and deletes the object. This way we can be sure the code doesn't
exist on the DB. It then tries to generate an attribute of that type, to
force the exception to be thrown.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>