[clang-tools-extra] [clangd] Test background index file contents (PR #179956)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

I have some question here:
- We already have background-index.test, which checks that go-to-definition request provides correct result with another file uri. Maybe background-index.test is enough to be sure that paths inside shards are correct?
- In you test we check that file names are correct, but not directories. Do we need to cover the case when directories change?
- I am not sure that adding the test as a separate patch before the functionality implementation makes sense. The community can object to this new functionality. It's more typical to add the functionality and the test in a single patch.

https://github.com/llvm/llvm-project/pull/179956