railties depends on actionpack, so put actionpack in the include path

railties/Rakefile

@@ -17,7 +17,13 @@ namespace :test do
     dir = ENV["TEST_DIR"] || "**"
     Dir["test/#{dir}/*_test.rb"].each do |file|
       next true if file.include?("fixtures")
-      ruby '-Itest', "-I#{File.dirname(__FILE__)}/../activesupport/lib", file
+      dash_i = [
+        'test',
+        'lib',
+        "#{File.dirname(__FILE__)}/../activesupport/lib",
+        "#{File.dirname(__FILE__)}/../actionpack/lib"
+      ]
+      ruby "-I#{dash_i.join ':'}", file
     end
   end
 end