Clean up all dead files inside `tests/ui/`
While rebasing #135860 I noticed that there are several dead `*.stderr` files inside `tests/ui/`.
When I checked thoroughly, I found 69 dead `*.$revision.stderr` files, 3 other dead `*.stderr` files and one dead `*.rs` file.
Prior to #134808, compiletest's `--bless` didn't remove dead `*.stderr` files when the set of revisions changed in any way (renamings, removals, additions, …) which explains their existence.
Regarding the dead `*.rs` file, that one was located inside an `auxiliary/` directory (together with a `*.stderr` file) despite not being meant to be an auxiliary file (it's not referenced by any `//@ aux-*`, it has an accompanying `*.stderr` file and it's obvious from looking at #111056 which added it). Ideally compiletest or tidy would forbid `*.std{out,err}` files inside `auxiliary/` dirs, that would've caught it. I moved it, updated it and turned it into a proper UI test.
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How to reproduce:
1. Run `rm tests/ui/**/*.stderr`
2. Run `./x test tests/ui --bless` (or similar)
3. Manually / semi-automatically go through all tests that were ignored (likely due to your OS etc. not matching) and restore any stderr files that were overzealously removed
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r? compiler
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