add a few regressions test from #2313

The code was contributed from Carson McDonald (@carsonmcdonald)

test/t/class.rb

@@ -375,6 +375,20 @@ assert('clone Class') do
   assert_true(Foo.clone.new.func)
 end
 
+assert('class definition in singleton class') do
+  class AClassS
+    class << self
+      class BClass
+      end
+
+      def iclass
+        BClass
+      end
+    end
+  end
+  assert_equal(Class, AClassS.iclass.class)
+end
+
 assert('class variable and class << self style class method') do
   class ClassVariableTest
     @@class_variable = "value"