The issue from #18003 hasn't been shown to be caused by PDF.js, but it
did surface that we don't have (direct) unit test coverage for the
`BaseException` class. This made it more difficult to prove that the
`stack` property was already available on exception instances, but more
importantly it caused the CI to be green even though the suggested
change would have caused the `stack` property to disappear.
To avoid future regressions, for e.g. similar changes or a rewrite from
a closure to a proper class, this commit introduces a dedicated unit
test for `BaseException` that asserts that our exception instances
indeed expose all expected properties.
6b237e3358