Jenkins seems to have tied itself into a knot over the switch from \TestingAccessWrapper to \Wikimedia\TestingAccessWrapper:
- extension tests ignore the extension's composer.json and just use core/vendor (see mw-fetch-composer-dev.sh)
- core patches run a couple extension tests as well, which fail
- DonationInterface tests run with 1.27 core. (The tests pass, I have no idea why.)
The first two should be solvable by splitting up the core patch. Not sure what should be done about DonationInterface though.