[clang-tools-extra] [clangd][Tweaks] Improve test infra for testing tweaks with an index (PR #160391)

[Christian Kandeler via cfe-commits]

https://github.com/ckandeler approved this pull request.

Sorry about the delay, didn't get the notification due to an email client hiccup.

https://github.com/llvm/llvm-project/pull/160391