Working around limitations in subpixel precision event handling.

The bug/limitation where browser supports subpixel precision for elements but not for dispatched events was found in both modern Chrome (http://crbug.com/396380) and Firefox (?) browsers.
(IE doesn't seem to be affected: before and including IE9 - no subpixel for elements and events; from ie10 - subpixel is supported for both elements and events).

This test was lucky so far (mostly?) and didn't hit this issue, until Chrome 37 that enabled subpixel text scaling by default.

This change makes *sure* elements have subpixel coordinates (if only browser supports'em) and then it makes sure this test doesn't fail because of that (while still testing selenium atoms).

While there, I moved asserts from event handlers into the normal test flow so jsunit can properly attribute assertion failures to specific test methods.