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.
Edited by Ben Boeckel