Fix #14084: attach the test div to documentElement, not body.

I refactored support tests so that they don't require body presence. The `boxSizing` and `ajax` support tests have been removed as all supported browsers have them true.

Why do we have `support.ajax` in the first place? Is it because it can be turned off in IE? I don't see why do we keep this test but maybe I'm missing something.

I'll push a separate analogous pull request for `1.x-master` as simple cherry-pick won't work in this case.

Once these 2 get merged, I'll work on dividing the support module accross modules that actually use particular tests and then the module will disappear. (I still need to figure out what to do with `test/unit/support` but that's a separate case).