The listeners collection test only needs to happen in `addingEventListener`.

`settingEventListener` is only called for existing listeners, so only
`addingEventListener` needs to check, and it can do it before
delegating.
Accept event listener state in `model/object`.
Add comments about future per-listener support in the kernel.
"preserve-script-closures" configuration option tests.
Tests for capturing listener IDs on add and removing listeners by ID.
Factor out the common descriptors in the node state tests.
Omit `getEvent` result fields that are missing or empty.
Tests for event changes.