Consider using `globalThis` in harness code

[Ms2ger]

We've historically used self, as that's available in all Web globals. We're now starting to add tests for ShadowRealms, which don't support self, but now globalThis exists. We could change the harness code to support both - maybe even with a fallback to self to keep pre-2020 browsers working.

[jimmywarting]

i say just rip the bandaid of.
switch to using globalThis. otherwise use a polyfill. self.globalThis = self

[ptomato]

As of the current state of whatwg/html#9893 we do propose to have self in ShadowRealms.