test: black box bench inputs

[robjtede]

doesn't change results much, but technically more correct

Timer precision: 41 ns
display                         fastest       │ slowest       │ median        │ mean          │ samples │ iters
├─ display_always_pad_custom    207.1 ns      │ 9.915 µs      │ 290.1 ns      │ 291.2 ns      │ 1000    │ 1000
├─ display_always_pad_standard  198.3 ns      │ 5.884 µs      │ 232.2 ns      │ 237.6 ns      │ 1000    │ 32000
├─ display_bytesize_custom      249.1 ns      │ 4.749 µs      │ 332.1 ns      │ 324.5 ns      │ 1000    │ 1000
├─ display_bytesize_standard    172.3 ns      │ 1.115 µs      │ 187.9 ns      │ 192 ns        │ 1000    │ 32000
╰─ display_inner_display        171 ns        │ 65.08 µs      │ 177.5 ns      │ 244.2 ns      │ 1000    │ 32000