Check `.fixed` paths' existence in `run_ui`
This PR adds a test to check that there exists a `.fixed` file for every `.stderr` file in `tests/ui` that mentions a `MachineApplicable` lint. The test leverages `compiletest-rs`'s `rustfix_coverage` option.
I tried to add `.fixed` files where they appeared to be missing. However, 38 exceptional `.rs` files remain. Several of those include comments indicating that they are exceptions, though not all do. Apologies, as I have not tried to associate the 38 files with GH issues. (I think that would be a lot of work, and I worry about linking the wrong issue.)
changelog: none
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