Fix extruderstack test on CI
Fix extruderstack test on CI

We do not need a complete CuraApplication just so registerExtruder
can work. Instead, we just mock away the extruder manager so the rest of
the test can proceed.

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