Following on from discussions in chat, @mattwire I was able to get stripe to the point where it's running and now just surfacing E_NOTICES (https://github.com/SemperIT/CiviCARROT/runs/4848384105?check_suite_focus=true#step:17:38).
The first commit is the full thing that includes some of your phpunit8 fixes you already have in another PR, but I needed in order to get past that. The total diff then removes those so is just the other changes needed, so then shouldn't conflict with your other PR. But then this PR in whole will fail tests.
FYI @KarinG