Check if Test::Unit is defined before adding backtrace filter.

railties/lib/generators/rails/app/templates/test/test_helper.rb

 ENV["RAILS_ENV"] = "test"
+require 'test/unit'
 require File.expand_path(File.dirname(__FILE__) + "/../config/environment")
 require 'rails/test_help'
 

railties/lib/rails/test_unit/railtie.rb

@@ -16,7 +16,7 @@ class TestUnitRailtie < Rails::Railtie
 
     initializer "test_unit.backtrace_cleaner" do
       # TODO: Figure out how to get the Rails::BacktraceFilter into minitest/unit
-      unless defined?(Minitest) || ENV['BACKTRACE']
+      if defined?(Test::Unit::Util::BacktraceFilter) && ENV['BACKTRACE'].nil?
         require 'rails/backtrace_cleaner'
         Test::Unit::Util::BacktraceFilter.module_eval { include Rails::BacktraceFilterForTestUnit }
       end

railties/test/application/test_test.rb

@@ -23,6 +23,23 @@ def test_truth
       run_test 'unit/foo_test.rb'
     end
 
+    # Run just in Ruby < 1.9
+    if defined?(Test::Unit::Util::BacktraceFilter)
+      test "adds backtrace cleaner" do
+        app_file 'test/unit/backtrace_test.rb', <<-RUBY
+          require 'test_helper'
+
+          class FooTest < ActiveSupport::TestCase
+            def test_truth
+              assert Test::Unit::Util::BacktraceFilter.ancestors.include?(Rails::BacktraceFilterForTestUnit)
+            end
+          end
+        RUBY
+
+        run_test 'unit/backtrace_test.rb'
+      end
+    end
+
     test "integration test" do
       controller 'posts', <<-RUBY
         class PostsController < ActionController::Base