Problem/Motivation
Since testbot only tests issues with the vendor versions in the composer lock file, we need a way to test against recent vendor updates in order to catch issues like #3039408: Updating twig/twig to v1.38.0 or v1.38.1 causes fatal error and #3039584: The "Symfony\Bridge\PsrHttpMessage\Factory\DiactorosFactory" class is deprecated since symfony/psr-http-message-bridge 1.2, use PsrHttpFactory instead.. If we enable Symfony 4 support in 8.8, we will need a way to test against that as well
Proposed resolution
Keep an RTBC issue with a patch to run composer update against the codebase prior to running the test suite.