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

models/fixtures/repository.yml

@@ -1634,3 +1634,16 @@
   is_private: true
   num_issues: 1
   status: 0
+
+-
+  id: 56
+  owner_id: 2
+  owner_name: user2
+  lower_name: readme-test
+  name: readme-test
+  default_branch: master
+  is_empty: false
+  is_archived: false
+  is_private: true
+  status: 0
+  num_issues: 0