Generate keep file in tmp/pids

Since 9c913116, if you run `rackup`
in a machine that doesn't have that folder, the server will not start.

This doesn't happen on `rails s` because we create that folder when
starting the server on
https://github.com/rails/rails/blob/8c8b02784a637570451d7810fd3dfed4eae9a77d/railties/lib/rails/commands/server/server_command.rb#L70-L74.

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -232,6 +232,7 @@ def system_test
 
     def tmp
       empty_directory_with_keep_file "tmp"
+      empty_directory_with_keep_file "tmp/pids"
       empty_directory "tmp/cache"
       empty_directory "tmp/cache/assets"
     end

railties/test/generators/app_generator_test.rb

@@ -950,6 +950,7 @@ def test_create_keeps
       test/helpers
       test/integration
       tmp
+      tmp/pids
     )
     folders_with_keep.each do |folder|
       assert_file("#{folder}/.keep")