Add test to reflect #1622 behaviour

test/source/_layouts/post/simple.html

@@ -0,0 +1 @@
+<<< {{ content }} >>>

test/test_site.rb

@@ -156,7 +156,7 @@ class TestSite < Test::Unit::TestCase
 
     should "read layouts" do
       @site.read_layouts
-      assert_equal ["default", "simple"].sort, @site.layouts.keys.sort
+      assert_equal ["default", "simple", "post/simple"].sort, @site.layouts.keys.sort
     end
 
     should "read posts" do