Revert integration test refactoring that caused app test regressions

Haven't diagnosed yet. No similarly failing tests in Rails to work from.

cc @tenderlove, @eileencodes

Revert "there is always an integration session, so remove the check"
Revert "lazily create the integration session"
Revert "use before_setup to set up test instance variables"

This reverts commits 4cf3b8ac, 303567e5, and fa634484.

actionpack/lib/action_dispatch/testing/integration.rb

@@ -388,16 +388,8 @@ module Runner
 
       APP_SESSIONS = {}
 
-      attr_reader :app
-
-      def before_setup
-        super
-        @app = nil
-        @integration_session = nil
-      end
-
-      def integration_session
-        @integration_session ||= create_session(app)
+      def app
+        @app ||= nil
       end
 
       # Reset the current session. This is useful for testing multiple sessions
@@ -425,6 +417,8 @@ def remove! # :nodoc:
       %w(get post patch put head delete cookies assigns
          xml_http_request xhr get_via_redirect post_via_redirect).each do |method|
         define_method(method) do |*args|
+          reset! unless integration_session
+
           # reset the html_document variable, except for cookies/assigns calls
           unless method == 'cookies' || method == 'assigns'
             @html_document = nil
@@ -456,16 +450,19 @@ def open_session
       # Copy the instance variables from the current session instance into the
       # test instance.
       def copy_session_variables! #:nodoc:
+        return unless integration_session
         @controller = @integration_session.controller
         @response   = @integration_session.response
         @request    = @integration_session.request
       end
 
       def default_url_options
+        reset! unless integration_session
         integration_session.default_url_options
       end
 
       def default_url_options=(options)
+        reset! unless integration_session
         integration_session.default_url_options = options
       end
 
@@ -475,6 +472,7 @@ def respond_to?(method, include_private = false)
 
       # Delegate unhandled messages to the current session instance.
       def method_missing(sym, *args, &block)
+        reset! unless integration_session
         if integration_session.respond_to?(sym)
           integration_session.__send__(sym, *args, &block).tap do
             copy_session_variables!
@@ -483,6 +481,11 @@ def method_missing(sym, *args, &block)
           super
         end
       end
+
+      private
+        def integration_session
+          @integration_session ||= nil
+        end
     end
   end
 
@@ -659,6 +662,7 @@ def app
     end
 
     def url_options
+      reset! unless integration_session
       integration_session.url_options
     end
 

actionpack/test/dispatch/response_test.rb

@@ -254,6 +254,10 @@ def test_response_body_encoding
 end
 
 class ResponseIntegrationTest < ActionDispatch::IntegrationTest
+  def app
+    @app
+  end
+
   test "response cache control from railsish app" do
     @app = lambda { |env|
       ActionDispatch::Response.new.tap { |resp|