[clang-tools-extra] [clang-doc] Avoid reading files in unit tests (PR #141269)

[Paul Kirth via cfe-commits]

ilovepi wrote:

### Merge activity

* **May 23, 6:45 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/141269).


https://github.com/llvm/llvm-project/pull/141269