MAGETWO-87963: Test testRouteConfigsValidation fails when first merged routes.xml contains >1 route element.

- Add router node

Author: viktym

dev/tests/integration/testsuite/Magento/Test/Integrity/Modular/RouteConfigFilesTest.php

@@ -61,7 +61,7 @@ public function testRouteConfigsValidation()
             $files = array_merge($files, glob($mask));
         }
         $mergedConfig = new \Magento\Framework\Config\Dom(
-            '<config></config>',
+            '<config><router/></config>',
             $this->validationStateMock,
             $this->_idAttributes
         );