Bug 34223: Add unit test for existing_statuses

Bug 34223: Add unit test for existing_statuses

This patch adds a unit test for the 'existing_statuses' method.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Laura Escamilla <laura.escamilla@bywatersolutions.com>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit cda13a73284ce049535f782dad794c6593e55ba9)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>