testutil: remove Diff and FetchTar
Created by: keegancsmith
These helpers where only used in one place:
- Diff can be replaced with cmp.Diff.
- FetchTarFromGithub was only used by searcher. So inlined.
Test Plan: CI since this only affects test code.
Created by: keegancsmith
These helpers where only used in one place:
Test Plan: CI since this only affects test code.