Test data: the "Car" class must have a "clone" function.

spec/test-data.cr

@@ -93,6 +93,8 @@ class Car
 	property color    : String        # a simple attribute (1-n relations)
 	property keywords : Array(String) # tags about a car, example: "shiny" (n-n relations)
 
+	def_clone
+
 	def initialize(@name, @color, @keywords)
 	end
 	class_getter cars = [