[clang-tools-extra] [clang-tidy][NFC] Add empty '.clang-tidy' file in tests dir to silent warnings in IDE (PR #171029)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> Will be good idea to change commit summary to end with `to silent warnings in IDE`.

Yes, also missed that my title was bad initially.

https://github.com/llvm/llvm-project/pull/171029