Isolate logic of Backstage passes in a method.
I realized that the updateQuality logic is divided in 2 main blocks, what happens before the decrease of the sell in, and what happens after. So, I wrote some methods that will help in the refactor of the updateQuality.
Implemented Conjured Cake (decrease quality twice as normal item), that explain the change in one test.

Implemented the division of updateQuality in 2 stages, before decrease the sell in of the item, and after.