[clang-tools-extra] [clang-tidy] Implement alphabetical order test (PR #166072)

[Baranov Victor via cfe-commits]

vbvictor wrote:

As for the test, it could be a simple `xxx_test.py` script, look at how tests in https://github.com/llvm/llvm-project/tree/main/.ci are organized. AFAIK they are not run in premerge CI, but for small tests I guess it's not needed that much.

As for the script name, we should remove "clang-tidy" part because it duplicates a part presented in filepath, so It should be `check-alphabetical-order.py` and `check-alphabetical-order_test.py` (reordered words a bit).



https://github.com/llvm/llvm-project/pull/166072