Add find_glyph_index benchmark regression test for `format=6` font
We have benchmark regression tests in benches/api.rs, that can be run with cargo +nightly bench
& cargo +nightly test --bench api
.
We're not currently testing format 6
find_glyph_index
logic. We should add new one covering this format, similar to how find_glyph_index_gudea
covers format 4
.