CI: phpunit should fail on deprecated notices

[kkmuffme]

If psalm itself triggers PHP notices in CI these aren't reported and the tests don't fail
e.g. #10950 that issue would have been caught 3 years ago and I wonder how nobody has encountered that notice when running CI locally with a stricter error handler :)

[psalm-github-bot]

Hey @kkmuffme, can you reproduce the issue on https://psalm.dev? These will be used as phpunit tests when implementing the feature or fixing this bug.