add Module#class_variables test

test/t/module.rb

@@ -31,6 +31,18 @@ assert('Module#append_features', '15.2.2.4.10') do
   Test4AppendFeatures2.const_get(:Const4AppendFeatures2) == Test4AppendFeatures2
 end
 
+assert('Module#class_variables', '15.2.2.4.19') do
+  class Test4ClassVariables1
+    @@var1 = 1
+    @@var2 = 2
+  end
+  class Test4ClassVariables2
+    @@var3 =2
+  end
+  Test4ClassVariables1.class_variables == [:@@var1, :@@var2] &&
+  Test4ClassVariables2.class_variables == [:@@var3]
+end
+
 assert('Module#const_defined?', '15.2.2.4.20') do
   module Test4ConstDefined
     Const4Test4ConstDefined = true