test: update external image test to use other placeholder service

 placekitten.com seems to fail at the moment so lets switch to another one.

tests/test-external-image.php

@@ -147,7 +147,7 @@ public function testExternalImageWithExternalUrl()
 
     public function testExternalImageWithExternalUrlAndNoImageExtension()
     {
-        $file = 'https://placekitten.com/222/333';
+        $file = 'https://via.placeholder.com/640x360';
         $filename = basename(Timber\ImageHelper::get_sideloaded_file_loc($file));
         $this->delete_existing_sideloaded_image($file);