DOM Events Test Suite Coverage

[Gabisampaio]

I measured the coverage of the DOM Events test suite against the standard and there seems to be a few coverage gaps. I list three scenarios below:

Scenario 1: Dispatch with optional flags

There seems to be no test calling the dispatchEvent function with the optional flags legacyTargetOverride and legacyOutputDidListenersThrow.
Additional test: calling dispatchEvent with optional flags activated and check whether handler is still triggered.

Scenario 2: Activation behaviour

Another potential scenario to cover is activation behaviour, which is triggered, for instance, if we call dispatchEvent on an element of type input.

Additional test: one could check if certain properties hold for the dispatch triggering activation behaviour, such as the event phase is set to NONE during the execution of a handler.

Scenario 3: Slottable feature

It could be interesting to write tests using the HTMLSlotElement.

Additional test: one could perhaps test if creating a slot on an input element and dispatching an event on the input element by calling dispatchEvent would trigger activation behaviour.

I appreciate any feedback regarding these scenarios. If necessary, I can submit additional tests via pull requests to improve the test suite coverage.