check limit of serialized command in unit tests

lib/private/command/queuebus.php

@@ -41,6 +41,9 @@ class QueueBus implements IBus {
 		if ($command instanceof ICommand) {
 			// ensure the command can be serialized
 			$serialized = serialize($command);
+			if(strlen($serialized) > 4000) {
+				throw new \InvalidArgumentException('Trying to push a command which serialized form can not be stored in the database (>4000 character)');
+			}
 			$unserialized = unserialize($serialized);
 			$unserialized->handle();
 		} else {