fixtures: add function to resolve sample shortened name by MD5

Ok so it looks like we need the opposite of `fixtures.get_sample_md5_by_name` e.g. `fixtures.get_sample_short_name_by_md5` or the like. Let's leave this code as-is for now and I'll open a separate issue to update the fixtures.

_Originally posted by @mike-hunhoff in https://github.com/mandiant/capa/pull/1727#discussion_r1300208729_

Start by locating fixtures.get_sample_md5_by_name and inspect how sample names and MD5 values are represented. Add the corresponding lookup from an MD5 to a sample's shortened name, with naming consistent with the existing helper. Done means callers can resolve a sample shortened name from its MD5.

Written by the indexing model from the issue text.