Fix stable clippy string repeat lint

[joostjager]

Uncaught by broken CI

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

This is a trivial, behavior-preserving change in test code. core::iter::repeat(c).take(n).collect::<String>() is equivalent to "c".repeat(n) for each case, including the multibyte emoji case where "\u{1F600}".repeat(n) produces n emoji characters identical to the iterator version.

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