Add ForHelper's for system tests

These FormHelpers are selectors that aren't a capybara default but are
considered useful for Rails applications.

actionpack/lib/system_testing/test_helper.rb

 require 'capybara/rails'
+require 'system_testing/test_helpers'
 
 module SystemTesting
   module TestHelper
     include Capybara::DSL
+    include TestHelpers::FormHelper
 
     def after_teardown
       Capybara.reset_sessions!

actionpack/lib/system_testing/test_helpers.rb

+module SystemTesting
+  module TestHelpers
+    extend ActiveSupport::Autoload
+
+    autoload :FormHelper
+  end
+end

actionpack/lib/system_testing/test_helpers/form_helper.rb

+module SystemTesting
+  module TestHelpers
+    module FormHelper
+      def fill_in_all_fields(fields)
+        fields.each do |name, value|
+          fill_in name, with: value
+        end
+      end
+
+      def click_checkbox_label(name, checked: false)
+        field = find_checkbox(name, checked)
+        label = find_label_wrapper(field)
+        label.click
+      end
+
+      def press_enter
+        page.driver.browser.action.send_keys(:enter).perform
+      end
+
+      private
+        def find_checkbox(name, checked)
+          find(:field, name, visible: :all, checked: checked)
+        end
+
+        def find_label_wrapper(field, location: './ancestor::label')
+          field.find :xpath, location
+        end
+    end
+  end
+end