[clang-tidy][NFC] Add empty '.clang-tidy' file in tests dir to silent warnings in IDE (#171029)
When working on tests, `clangd` with option `--clang-tidy` will report
warnings from
[root](https://github.com/llvm/llvm-project/blob/main/.clang-tidy)
clang-tidy config.
I believe these warnings serve no purpose in tests, so we better disable
them to silent warnings in IDE.