Add regression test for $-prefixed fragment specifier in repetition

[asuto15]

Fixes #157157.

The diagnostic was fixed in #155643, but the existing test only covers an accidental $ before a fragment specifier at the top level of a matcher.
This adds the equivalent case inside a repetition.

The new case reuses $test:$tt from the adjacent test so that repetition nesting is the only difference between the two cases.

Tested with:

• ./x test tests/ui/macros/macro-missing-fragment.rs

This is my first contribution to this repository, so please let me know if I have missed any conventions or required steps or should make any additional changes.

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r? @TaKO8Ki

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[chenyukang]

Thanks!
@bors r=chenyukang

[rust-bors]

📌 Commit d905d0a has been approved by chenyukang

It is now in the queue for this repository.