29204 add cart test

@prabinv @texasag @kevinsproles cc: @tsanko & @tylertadej 

This pull requests adds a mock library for api calls and several tests related to the vnCart service
- saveCart with success
- save cart with warnings
- saveCart with errors
- init cart (with empty cart response)

This branch was rebased with sprint 5. I am not sure why github cannot merge them.