Add some basic manual tests for Gamepad API.

I added some very simple manual tests for the Gamepad API. I've run them in Firefox and Chrome. They seem to not work 100% reliably on Chrome when run via /runner/, I'm not sure if that's a bug in Chrome or if the tests need a change to be more reliable. I'll poke a Chrome engineer on that later. When run standalone all 3 tests pass in Chrome, and 2/3 pass in Firefox (Firefox doesn't have Gamepad.timestamp yet).

I'm planning on writing some more tests, specifically around currently underspecified behavior in the spec that needs to be clarified, but this is my first time writing web-platform-tests so I wanted to make sure I'm not doing anything unreasonable first.