Pig Latin: Added tests and improved test descriptions (exercism#390)

* Improved test descriptions

Some test descriptions weren't doing a good job of communicating
the rules of Pig Latin, leading to implementations that gamed the tests.
For example, implementations were looking specifically for a leading 'yt' word as
a vowel word, but not any other combination of 'y' and a consonant, like 'yd'.

A couple tests are added to break the overly-specific implementations.

The following implmentation passes the new version of the tests:

http://exercism.io/submissions/2ec927894ece47baa8fe482f9f210a63

* Revert 'qat' test change

* Make example.exs test pass

exercises/pig-latin/example.exs

@@ -32,12 +32,11 @@ defmodule PigLatin do
   end
 
   defp consonant_prefix_and_rest(word) do
-    if Regex.match?(~r/^yt|xr/, word) do
+    if Regex.match?(~r/^[yx][bcdfghjklmnpqrstvwxy]+/, word) do
       ["", word]
     else
       ~r/^(s?qu|(?:[^aeiou]*))?([aeiou].*)$/
       |> Regex.run(word, capture: :all_but_first)
     end
   end
 end
-

exercises/pig-latin/pig_latin_test.exs

@@ -82,7 +82,7 @@ defmodule PigLatinTest do
     end
   end
 
-  describe "some letter clusters are treated like a single consonant" do
+  describe "consecutive consonants are treated like a single consonant" do
     @tag :pending
     test "word beginning with ch" do
       assert PigLatin.translate("chair") == "airchay"
@@ -114,16 +114,26 @@ defmodule PigLatinTest do
     end
   end
 
-  describe "some letter clusters are treated like a single vowel" do
+  describe "'x' and 'y', when followed by a consonant, are treated like a vowel" do
     @tag :pending
-    test "word beginning with yt" do
+    test "word beginning with y, followed by a consonant" do
       assert PigLatin.translate("yttria") == "yttriaay"
     end
 
+    @tag :pending
+    test "word beginning with y, followed by another consonant" do
+      assert PigLatin.translate("yddria") == "yddriaay"
+    end
+
     @tag :pending
     test "word beginning with xr" do
       assert PigLatin.translate("xray") == "xrayay"
     end
+
+    @tag :pending
+    test "word beginning with xb" do
+      assert PigLatin.translate("xbot") == "xbotay"
+    end
   end
 
   describe "phrases are translated" do
@@ -133,4 +143,3 @@ defmodule PigLatinTest do
     end
   end
 end
-