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/repo_unit.yml

@@ -569,3 +569,9 @@
   type: 3
   config: "{\"IgnoreWhitespaceConflicts\":false,\"AllowMerge\":true,\"AllowRebase\":true,\"AllowRebaseMerge\":true,\"AllowSquash\":true}"
   created_unix: 946684810
+
+-
+  id: 84
+  repo_id: 56
+  type: 1
+  created_unix: 946684810

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

models/fixtures/user.yml

@@ -66,7 +66,7 @@
   num_followers: 2
   num_following: 1
   num_stars: 2
-  num_repos: 11
+  num_repos: 12
   num_teams: 0
   num_members: 0
   visibility: 0