Make tests safer

Using a class variable here makes the tests susceptible to phantom
greens because (depending on the order of the tests), the class variable
could have been set by a prior test. (Test polution)

Better to have the variable be tied to the instance.

activesupport/test/concern_test.rb

@@ -13,11 +13,11 @@ class ConcernTest < ActiveSupport::TestCase
       end
 
       def included_ran=(value)
-        @@included_ran = value
+        @included_ran = value
       end
 
       def included_ran
-        @@included_ran
+        @included_ran
       end
     end