Add __forceCheckValidity and test

[yuriy-fix]

Connected to vaadin/vaadin-date-picker#696.

In many cases the component containing text-field (i.e.) faces a problem with the validation of internal input, because checkValidity of the text-field returns false based on the previous value of the invalid property. (return !this.invalid;) There wasn’t any simple way of making the field checkValidity without setting validation constraints.

This PR adds this possibility.

[web-padawan]

🙈 LGTM

[web-padawan]

Please update sinonjs to fix the bower installation.

[yuriy-fix]

There is also possibility to go with:

checkValidity() {
  if (this.required || this.pattern || this.maxlength || this.minlength || this.__checkValidityForced) {
    this.__checkValidityForced = false;
    return this.inputElement.checkValidity();
  } else {
    return !this.invalid;
  }
}

__forceCheckValidity() {
  this.__checkValidityForced = true;
}