eliminated unit test reliance on Shop interface

The unit tests must interface with the class/object/api they are testing, but hard-coding values in the unit tests will only make changes more difficult if and when the interface changes because then the change would have to be applied to each and every unit test. Instead, I am creating a local consistent function to wrap around a function of the interface. Therefore, if the interface changes, the change need only be applied to the wrapper function, not every unit test.