core-input test added and applied workarounds to other tests

A few basic tests for core-input
applied the suggested workarounds and changes
It works without `Platform.flush()`, so I will just remove the comments/TODOs

`onMutation` was in polymer-elements tests. The test didn't work this way anymore in core-elements and the JS tests have removed these lines too. I didn't question the change in JS or investigate further.