github/test: use `iter::repeat_n` to build example string

A recent change to Rust[1] started using u16 as the size for width formatting internally. This ended up tripping the test code which uses width formatting to make a just-too-long string right at the u16::MAX threshold. Replace with iterator construction instead.

Update the minimum to 1.82 where iter::repeat_n was stabilized.

[1] https://github.com/rust-lang/rust/pull/136932