Add a test expectations file for to-upstream tests #3835

Sometimes when investigating issues tagged "maybe fixed (add tests to confirm)", I write a test and find that it fails. At that point, there's nothing in the current project that is set up to accept that test, so we just throw it away and the investigation progress is lost.

Instead, we should have a list (similar to the to-upstream.yml file) which lists expected-failing to-upstream tests. Then I can check in failing to-upstream tests, and we can use them to track the issues.

The existing field for comments should always contain a jsdom issue in this file, probably.