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.