refactor: extract file-gathering into new internal FileSpec API#174

ErichDonGubler merged 2 commits intomainerichdongubler-mozilla/moz-webgpu-cts:mainfrom

erichdongubler/push-xmtktnnvrwolerichdongubler-mozilla/moz-webgpu-cts:erichdongubler/push-xmtktnnvrwolCopy head branch name to clipboard