If uncaught exceptions occur in the tests (which happened in #17962 and
can be triggered manually by throwing an error in `integration-boot.js`)
the teardown logic of the tests doesn't get to run and thus spawned
browser processes are not closed properly. Given that `test.mjs` is the
only process that has a reference to them they will become orphaned and
keep running if `test.mjs` exits without explicitly closing them.
This commit fixes the issue by always closing the browsers if uncaught
exceptions occur, and we make sure to log them for debugging purposes.
3afe2d3048