Allow mocking a tap on given xy-coordinates

[NovapaX]

Currently it's not possible to easily simulate a click / tap on given coordinates.

see also: https://github.com/PolymerElements/iron-test-helpers/pull/93#issuecomment-367854446

I'd expect to be able to call: tap(node, xy) like the click function (which is not exposed):

function tap(node, xy) {
    xy = xy || middleOfNode(node);
    // do stuff
}