[clang] [clang-format] Add test to ensure formatting options docs are updated (PR #118154)

[Owen Pan via cfe-commits]

owenca wrote:

> When running the new testcase the file plurals.txt, which is in the repo, is opened for writing? Is this really desired?

Fixed in 74d29c6393df606e98e7c42b9a97f56f335e3ffb.

https://github.com/llvm/llvm-project/pull/118154