When the test fails, upgrade the source to pass in those data:

  def decode(ypath)
    ypath =~ /^node(:.*)/
    return $1.gsub(':', '//')
  end
...

        href = {
          :id => 'link_'+ypath,
          :url => {
            :action    => 'display_iframe',
            :page_name => page_name,
            :ypath     => decode(ypath)
            } }
        @x << @page.link_to_remote($1, href)

That triggers an error message, which tells us how to upgrade the test:

        assert_xpath 'Statement[1]//ArgumentList[1]' do |node|
          assert_equal
'/wiki/display_iframe?ypath=%2F%2Ftest_uncle_wiggily&page_name=WikiTestPage',
                       assert_argument(1)
        end

The front-slashes were predictably escaped as %2F, and the arguments arrived in a different order.

Patching that test case isn't good enough. For every feature in our code, including the systems that generate a URI's path and query segments, we need a matching feature on the test side:

  def assert_params(path_query)
    path, query = path_query.split('?')
    params = {}

    if query
      query.split('&').each do |item|
        key, value = item.split('=')
        params[key.to_sym] = CGI::unescape(value)
      end
    end

    return [path, params]
  end

  def test_test_nodes_link_to_display_iframe
    render_wiki(get_omap)

    assert_xpath '//strong/a[ . = "test" ]' do |a|
      assert_equal '#', a.href
      assert_js 'ajax = ' + a.onclick do
        assert_xpath 'Statement[1]//ArgumentList[1]' do
          command, params = assert_params(assert_argument(1))
          assert_equal '/wiki/display_iframe', command
          assert_equal '//test_uncle_wiggily', params[:ypath]
          assert_equal 'WikiTestPage', params[:page_name]
        end
      end
    end
  end

And that system looks very easy to change, upgrade, and reuse. It won't break when Ruby's beloved Hash objects decide to change the order of their keys.