Logic for adding items and validating them

tests.rb

@@ -0,0 +1,22 @@
+require "minitest/autorun"
+
+require "./checkout.rb"
+
+class TestCheckout < Minitest::Test
+  def setup
+    @checkout = Checkout.new
+  end
+
+  def test_that_unknown_items_cannot_be_added
+    assert_raises Checkout::InvalidItem do
+      @checkout.add(:foo)
+    end
+  end
+
+  def test_that_allowed_items_are_added
+    @checkout.add(:PI5)
+    @checkout.add(:ZERO2W)
+
+    assert_equal [:PI5, :ZERO2W].sort, @checkout.items.sort
+  end
+end