tests: fix cppcheck findings

- bitarithm_timings: use more void, decrease range
- struct_tm_utility: add cppcheck-suppress invalidscanf

I'm not sure I'm happy with this, I'd just like to add context to #1432
Yeah, vim is by default usually very bad when it comes to indent statements that span over multiple lines. I would rather leave it as is or align the following lines with the second `(`.