[clangd] Remove ScratchFS from tests

This can lead to issues if files in the tmp directory we don't care about / control are found.
This was partially addressed in D94321, but this is a more permanent fix.
Fixes https://github.com/clangd/clangd/issues/354

Reviewed By: adamcz, sammccall

Differential Revision: https://reviews.llvm.org/D94359