Support: Simplify the box-sizing test

Don't require body in the support.boxSizing test.

I don't know why the test was so complicated, it seems like it was done on purpose which suggests that some implementation could pass the test from this PR but would fail the current one. However, all our supported browsers match what's expected for `support.boxSizing` so it seems if there was ever any issue with that, it's now resolved.

Apart from not requiring a body (and layout), this PR also saves 26 bytes.