Test renderReadmeFile (#23185)

Add test coverage to the important features of
[`routers.web.repo.renderReadmeFile`](067b0c2664/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

tests/gitea-repositories-meta/user2/readme-test.git/objects/3b/23d7f1a9cb904cb46f5f2272bfa5ed5f871fb9

@@ -0,0 +1 @@
+x
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