Problem/Motivation
In #3475974: Add www-data user to root group so all env vars are passed to run-tests.sh there's work ongoing to allow all environment variables to be visible to test runners during test execution.
run-tests.sh fails when using sudo -E, that would allow passing the current environment variables.
Original post
In #3475974: Add www-data user to root group so all env vars are passed to run-tests.sh there's work ongoing to allow dropping usage of sudo to run tests. sudo prevents passing the current environment variables to the runners, and there are some that are now used for configuring PHPUnit extensions behaviour.
It would be good to try that in core, too, and in such occasion fix any permission issues with www-data.
Proposed resolution
Fix run-tests.sh bug, and adjust CI jobs to use sudo -E as appropriate.