Fix test_setVariantByIdExists
Fix test_setVariantByIdExists

It now also uses the return_value construct.

Contributes to issue CURA-3497.

ynotstartups/Wanhao,hmflash/Cura,fieldOfView/Cura,ynotstartups/Wanhao,fieldOfView/Cura,Curahelper/Cura,Curahelper/Cura,hmflash/Cura