Extract and automate test cases

[frivoal]

The test cases in the repo cover a lot of scenario, but they're set up to be manual tests. We should refactor them and automated them as far as possible into something that could be submitted to web-platform-tests

[jihyerish]

The test cases in the web platform test format have been added in

• https://github.com/WICG/spatial-navigation/tree/master/tests

, both for spec and UX point of view.

Specifically for the expected result of UX test cases, we need the review from others (other browser vendors or developers).