Fix malformed error annotations in a UI test

[fmease]

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.

[rustbot]

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[oli-obk]

r? @oli-obk @bors r+ rollup

[bors]

📌 Commit 43b729d has been approved by oli-obk

It is now in the queue for this repository.