tests: fix priceTest3 to run the right test
tests: more comments
tests: add tests for set size 3 promotion

Testing for promoting size 3 sets to size 4 by moving a item from set of size 5.
Fixed priceTest4 and takeSetsTest4

This was done by implementing set size fixing by promoting sets of size 3 to
size of size 4 if sets of size 5 are available.

This is done because it maximizes the discount amount.
Added a description file for itemsale.hs