Tidy check for test revisions that are mentioned but not declared
If a `[revision]` name appears in a test header directive or error annotation, but isn't declared in the `//@ revisions:` header, that is almost always a mistake.
In cases where a revision needs to be temporarily disabled, adding it to an `//@ unused-revision-names:` header will suppress these checks for that name.
Adding the wildcard name `*` to the unused list will suppress these checks for the entire file.
(None of the tests actually use `*`; it's just there because it was easy to add and could be handy as an escape hatch when dealing with other problems.)
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Most of the existing problems discovered by this check were fairly straightforward to fix (or ignore); the trickiest cases are in `borrowck` tests.
8c7c151a7a