Add a couple of extra test cases

tests.rb

@@ -13,6 +13,12 @@ class TestCheckout < Minitest::Test
     end
   end
 
+  def test_that_nil_is_not_allowed_to_be_added
+    assert_raises Checkout::InvalidItem do
+      @checkout.add(nil)
+    end
+  end
+
   def test_that_allowed_items_are_added
     @checkout.add(:PI5)
     @checkout.add(:ZERO2W)
@@ -84,7 +90,7 @@ class TestCheckout < Minitest::Test
 
   def test_that_price_is_not_reduced_for_less_than_3_picos
     2.times do
-      @checkout.add(:PICO2)
+      @checkout.add(:PICO2) # 4.00 * 2 = 8
     end
 
     assert_equal 8.00, @checkout.total
@@ -92,13 +98,31 @@ class TestCheckout < Minitest::Test
 
   def test_that_both_discounts_apply_at_the_same_time
     4.times do
-      @checkout.add(:ZERO2W) # 30.00 total
+      @checkout.add(:ZERO2W) # 30.00 with BOGOF
     end
 
     3.times do
-      @checkout.add(:PICO2) # 10.50 total
+      @checkout.add(:PICO2) # 10.50 with bulk discount
     end
 
     assert_equal (30.00 + 10.50), @checkout.total
   end
+
+  def test_that_total_is_correctly_recalculated_when_items_are_added
+    2.times do
+      @checkout.add(:PICO2)
+    end
+
+    assert_equal 8.00, @checkout.total
+
+    @checkout.add(:PICO2) # 3 in total now, so discount applies
+
+    assert_equal 10.50, @checkout.total
+
+    4.times do
+      @checkout.add(:ZERO2W) # 30.00 with BOGOF
+    end
+
+    assert_equal 40.50, @checkout.total
+  end
 end