Rollup merge of #93953 - jackh726:known_bug, r=Mark-Simulacrum

Add the `known-bug` test directive, use it, and do some cleanup

cc rust-lang/compiler-team#476

Now tests can be annotated with `known-bug`, which should indicate that the test *should* pass (or at least that the current output is a bug). Adding it relaxes the requirement to add error annotations to the test (though it is still allowed). In the future, this could be extended with further relaxations - with the goal to make adding these tests need minimal effort.

I've used this attribute for the GAT tests added in #93757.

Finally, I've also cleaned up `header.rs` in compiletest a bit, by extracting out a bit of common logic. I've also split out some of the directives into their own consts. This removes a lot of very similar functions from `Config` and makes `TestProps::load_from` read nicer.

I've split these into separate commits, so I in theory could split these into separate PRs if they're controversial, but I think they're pretty straightforward.

r? ``@Mark-Simulacrum``

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