[clang-tools-extra] [clang-tidy][test] Make check_clang_tidy.py work with very long file paths (PR #155318)

[Nico Weber via cfe-commits]

nico wrote:

This is fine, but IMHO we should also rename the test to something shorter. Long paths are a pain in explorer.exe and other places too.

https://github.com/llvm/llvm-project/pull/155318