fix(combobox): update internal state after custom value is added

[jcfranco]

Related Issue: #10731, #11382

Summary

This updates the combobox internal state after an custom item is added.

Note: this also adds createEventTimePropValuesAsserter to help assert on event-emit time component state.

[driskull]

LGTM! updateItems being called on adding chip should be fine since it doesn't occur often.

[jcfranco]

@driskull Agreed. We might need to refactor combobox to update state in a more granular fashion. E.g., adding a custom item should update internal items and its filtering data, but probably doesn't need to filter at that time.