Recently I've discovered that we have https://github.com/wiredtiger/mongo-tests repo that contains huge amount of different tests. Different tests have different quality and some of them are probably useless but some looks really promising. The only thing I know now is that we regularly run many-collection-test from this repo as  a part of our perf testing.

That said, this looks like a potential quick win from a testing perspective if we can identify additional test cases there that could be integrated into our regular testing workflows with relatively low effort. At the very least, we should probably restructure the repository to clearly separate the tests we still consider useful from the legacy tests that we no longer plan to maintain or use.