output.html

<div class="field">
  <header class="field-header">
    <label class="field-label" for="wnw_form_builder_test_model_name">
      Name <small>(required)</small>
    </label>
  </header>
  <div class="field-feedback">
    <span class="field-errors"></span>
    <span class="field-success"></span>
    <span class="field-warning"></span>
  </div>
  content
</div>

test.rb

# <div> that wraps everything
assert_select document, wrapper_selector do |wrapper|
  # <header> that wraps label
  assert_select wrapper, "header.#{default_classes(:header)}" do |header|
    # label has classes and `for` attribute
    assert_select header, "label.#{default_classes(:label)}" do |label|
      # field required output
      assert_select label, "small" do |small|
        # actual "(required)" text
        assert_equal I18n.t('forms.field_required'), small.children.first.text
      end
      # label text
      assert_equal 'Name', label.children.first.text
    end
  end
  # validation error output
  assert_select wrapper, error_selector
  # validation success output
  assert_select wrapper, success_selector
  # actual content of field
  assert_equal 'content', wrapper.children.last.text
end