I don't understand why these tests are failing, or even why they're successful:
https://integration.wikimedia.org/ci/job/mwext-testextension-hhvm-non-voting/1187/console
It seems to be arbitrary per integration worker whether the extension's vendor/ libraries are installed or not. My blind guess is that we might be updating vendor when composer.json changes, but we're not refreshing vendor when the patch under test doesn't change composer.json?