add test for donut_rating.average
add test for donut_rating.average

CorainChicago/DonutDatabase,CorainChicago/DonutDatabase,CorainChicago/ChicagoDonutClub,CorainChicago/DonutDatabase,CorainChicago/ChicagoDonutClub,CorainChicago/ChicagoDonutClub