use_transactional_fixtures= and Test::Rails

With a normal test_helper, everything works just fine. However, if I change
test_helper.rb to include test/rails and the class from Test::Unit::TestCase
to Test::Rails::TestCase, I get this:

./test/functional/../test_helper.rb:19: undefined method
`use_transactional_fixtures=' for Test::Rails::TestCase:Class
(NoMethodError)
        from ./test/functional/contact_controller_test.rb:1
        from
/usr/local/lib/ruby/gems/1.8/gems/rake-0.7.1/lib/rake/rake_test_loader.rb:5
        from
/usr/local/lib/ruby/gems/1.8/gems/rake-0.7.1/lib/rake/rake_test_loader.rb:5

I understand that this can be the result of a missing gem, so I wrote a
quick program to find unique required files outside of vendor/rails and
couldn't find a missing gem.

Does anyone have any insight into this?