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/21/470f9b3e8ff24e0fa083d2dbc447f4c3401355

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

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tests/gitea-repositories-meta/user2/readme-test.git/objects/60/ea618ae7d4ecbe9c1962591c7da1b05bb1a5c8

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tests/gitea-repositories-meta/user2/readme-test.git/objects/e3/a6fd8fe49e323ee10017f72b777a53fbd8076f

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tests/gitea-repositories-meta/user2/readme-test.git/objects/info/packs

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