Avoid race condition in AJ integration tests

Make sure the file doesn't exist until we've finished writing it.

activejob/test/support/integration/dummy_app_template.rb

@@ -18,12 +18,13 @@ class TestJob < ActiveJob::Base
   queue_as :integration_tests
 
   def perform(x)
-    File.open(Rails.root.join("tmp/\#{x}"), "wb+") do |f|
+    File.open(Rails.root.join("tmp/\#{x}.new"), "wb+") do |f|
       f.write Marshal.dump({
         "locale" => I18n.locale.to_s || "en",
         "executed_at" => Time.now.to_r
       })
     end
+    File.rename(Rails.root.join("tmp/\#{x}.new"), Rails.root.join("tmp/\#{x}"))
   end
 end
 CODE

activejob/test/support/integration/helper.rb

@@ -5,6 +5,7 @@ ActiveJob::Base.queue_name_prefix = nil
 
 require "rails/generators/rails/app/app_generator"
 
+require "tmpdir"
 dummy_app_path     = Dir.mktmpdir + "/dummy"
 dummy_app_template = File.expand_path("../dummy_app_template.rb",  __FILE__)
 args = Rails::Generators::ARGVScrubber.new(["new", dummy_app_path, "--skip-gemfile", "--skip-bundle",