no such file to load test_helper

When running unit test individually via

$ ruby test/unit/city_test.rb

I get:

test/unit/city_test.rb:1:in `require': no such file to load -- test_helper (LoadError)
    from test/unit/city_test.rb:1:in `<main>'

$ ruby -v
ruby 1.9.2dev (2010-07-02 revision 28524) [x86_64-darwin10.4.0]
$ rails -v
Rails 3.0.0.beta4

in city_test.rb I have:

require "test_helper"

class CityTest < ActiveSupport::TestCase
  test "the truth" do
    assert false
  end
end

i've tried variations of using require "../test_helper", adding the .rb extension and adding File.dirname(FILE) to the front of the require and same results.