[clang] [C++20][Modules][Serialization] Add an additional test case for #120277. (PR #126349)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

It is easier to debug without `cd %t` generally.

https://github.com/llvm/llvm-project/pull/126349