Added the first multitask test

This one has an emphasis on the customer paying (i.e., the Market
interactions were built on top of the customer interaction tests)
Added in the second mixed test, and it worked.

At this point, with the market interaction emphasized test, I have come
to believe that my cashier works fully for its functionality.

I'm done unit testing, I'll check over my currrent code and make
modification to match any changes in my old code, but no more new code.