[clang] [clang-format] Extract utility functions outside of test fixture (PR #78108)

[via cfe-commits]

seranu wrote:

> I don't understand why we need to refactor this?

During the review for https://github.com/llvm/llvm-project/pull/77918 there was an ask to extract the refactor in a separate PR.

https://github.com/llvm/llvm-project/pull/78108