Make sure AS test env doesn't have duplicate libs in load path

activesupport/test/callback_inheritance_test.rb

 require 'test/unit'
-$:.unshift "#{File.dirname(__FILE__)}/../lib"
 require 'active_support'
 
 class GrandParent

activesupport/test/callbacks_test.rb

 # require 'abstract_unit'
 require 'test/unit'
-$:.unshift "#{File.dirname(__FILE__)}/../lib"
 require 'active_support'
 
 module CallbacksTest

activesupport/test/load_paths_test.rb

+require 'abstract_unit'
+
+class LoadPathsTest < Test::Unit::TestCase
+  def test_uniq_load_paths
+    load_paths_count = $LOAD_PATH.inject({}) { |paths, path|
+      expanded_path = File.expand_path(path)
+      paths[expanded_path] ||= 0
+      paths[expanded_path] += 1
+      paths
+    }
+
+    assert_equal [], load_paths_count.select { |k, v| v > 1 }, $LOAD_PATH.inspect
+  end
+end