`libregrtest` f-string update

[lczyk]

While looking at #129363 i've noticed a bit inconsistent string formatting (mix of .format, % and f-strings). This is an issue for a PR updating this.

Linked PRs

• gh-129370: Update libregrtest to use f-strings #129371

[encukou]

As far as I can see, there is no bug to fix here.
Please don't send PRs that are purely cosmetic; they're not worth the churn.

[lczyk]

@encukou gochya. 👍 sorry for the noise. won't happen again.