How to Maintain Integration Tests in edX

EdX is an open source Django project with 400,000 lines of code and a lot of tests. The edX organization on GitHub has 300 repos so there's probably at least a million and a half lines of code. We've got really a pretty extensive test suite and we're constantly re-jigging the sharding of tests across GitHub action suites to keep the total wall time of running the tests to a reasonable limit. I think right now it's about half an hour to run them all.