Add protection against classloader leaks to integration test flows

[holly-cummins]

@Sanne says:

I'd love it if all our live-reload classloaders, especially the ones used in integration tests, had some smartness (to be developed, I only have some rough ideas) to catch classloader leaks triggered by live-reload. We need our ITs to proactively defend against them.

FWIW I do know how to catch them in general - I just never had the time to integrate my POC within our classloaders, mainly because they are doing many other weird things which I wasn't following :)