Workaround doctrine serialisation issue in integration tests

Doctrine uses a default ProxyClassNameResolver but it is implemented
as an anonymous class and can not be serialised. This commit adds a custom
ProxyClassNameResolver that copies the anonymous one so that we avoid using the
anonymous fallback.
[MAILPOET-3658]

lib/Doctrine/TablePrefixMetadataFactory.php

@@ -22,6 +22,7 @@ class TablePrefixMetadataFactory extends ClassMetadataFactory {
       throw new \RuntimeException('DB table prefix not initialized');
     }
     $this->prefix = Env::$dbPrefix;
+    $this->setProxyClassNameResolver(new ProxyClassNameResolver());
   }
 
   public function getMetadataFor($className) {