Test renderReadmeFile (#23185)

Test renderReadmeFile (#23185)

Add test coverage to the important features of
[`routers.web.repo.renderReadmeFile`](https://github.com/go-gitea/gitea/blob/067b0c2664d127c552ccdfd264257caca4907a77/routers/web/repo/view.go#L273);
namely that:

- it can handle looking in docs/, .gitea/, and .github/
- it can handle choosing between multiple competing READMEs
- it prefers the localized README to the markdown README to the
plaintext README
- it can handle broken symlinks when processing all the options
- it uses the name of the symlink, not the name of the target of the
symlink