Add tests to ComonSpec.js

isLeapYear() method should return false, if year less than zero
getYearsBetweenDates() should return 19, if start date = (10)-1-1 and
end date = (-10)-1-1
getYearsBetweenDates() should return 19, if start date = (-10)-1-1 and
end date = (10)-1-1