Fail early if @position-try is not supported

[foolip]

No description provided.

[foolip]

I'm doing this because the test fails with an unclear harness error in Firefox and Safari:
https://wpt.fyi/results/css/css-anchor-position/at-position-try-invalidation-shadow-dom.html?run_id=5200154028408832&run_id=5067916582322176&run_id=5983905834598400&run_id=5118776041537536

It's better if none of the tests run and the error is clear.

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[foolip]

@lilles what do you think of this refactoring? You can see the results here:

https://wpt.fyi/results/css/css-anchor-position/at-position-try-invalidation-shadow-dom.html?label=pr_head&max-count=1&pr=50552

It makes the results a lot less readable at a glance, since there's just one subtest with a unhelpful name.

The key thing I want to achieve is no harness error in Firefox and Safari, which you can see here:
https://wpt.fyi/results/css/css-anchor-position/at-position-try-invalidation-shadow-dom.html?run_id=5200154028408832&run_id=5067916582322176&run_id=5983905834598400&run_id=5118776041537536

I think it would also be good if there aren't any passing subtests if @position-try isn't supported at all, although that's just a side benefit.

There are other ways of doing it, perhaps most obviously by putting the insertRule call inside the 3rd subtest. That would leave two passing subtests, and to avoid that we'd have to sprinkle in some assert_implements in the first two subtests...