Add new koans for comprehensions

Added to partially complete the "Koans for special forms" issue.
https://github.com/elixirkoans/elixir-koans/issues/53

test/koans/comprehensions_koans_test.exs

@@ -0,0 +1,18 @@
+defmodule ComprehensionsTests do
+  use ExUnit.Case
+  import TestHarness
+
+  test "Comprehensions" do
+    answers = [
+      [1, 4, 9, 16],
+      [1, 4, 9, 16],
+      ["Hello World", "Apple Pie"],
+      ["2 dogs", "2 cats", "4 dogs", "4 cats"],
+      [4, 5, 6],
+      ["Apple Pie", "Pecan Pie", "Pumpkin Pie"],
+    ]
+
+    test_all(Comprehensions, answers)
+  end
+
+end