Use querySelector(':checked') instead of selectedOptions

[Rich-Harris]

From Gitter — it seems selectedOptions isn't supported in IE, and using querySelector means we could enable <select> tests without waiting for a new version of JSDOM.

This is the relevant file.

Thanks for the info @Zirro!

[Zirro]

I believe I'll have time to submit a PR for this tomorrow. Some of the disabled tests need minor tweaks to pass (not directly related to the selectedOptions/:checked-replacement).