clean up dev-server packs

Author: rmosolgo

test/react/server_rendering/webpacker_manifest_container_test.rb

@@ -65,6 +65,8 @@ def test_it_loads_from_webpack_dev_server
     ensure
       Process.kill(9, webpack_dev_server)
       Process.wait
+      # Remove the dev-server packs:
+      WebpackerHelpers.clear_webpacker_packs
     end
   end
 end