Rollup merge of #136403 - fmease:fix-a-ui-test, r=oli-obk

Fix malformed error annotations in a UI test

The compiletest DSL still features a historical remnant from the time when its directives were merely prefixed with `//` instead of `//`@`` when unknown directive names weren't rejected since they could just as well be part of prose:

As an "optimization", it stops looking for directives once it stumbles upon a line which starts with either `fn` or `mod`. This allowed a malformed error annotation of the form `//`@[…]~^^^`` to go undetected & unexercised (as it's placed below `fn main() {`).

Obviously a character other than ``@`` would've mangled the error annotation, too (but it might've caught the reviewer's eye). I specifically found this file because I ran `rg '^(fn|mod)[\s\S]*?//`@'` tests/ui --multiline -trust` to check how footgun-y that "special feature" of compiletest is.

Author: matthiaskrgr