fix(input): improve html5 validation support

This CL improves mocking support for HTML5 validation, fixes the behaviour which invokes validators.

Previously, an input would only be revalidated if either its value changed, or if it was the empty
string but did not suffer from bad input --- now, it will be revalidated if either the value has
changed, or the value is the empty string, there is a ValidityState for the element, and that
ValidityState is being tested by one of the validators in the pipeline.

Closes #7937
Closes #7957