[clangd] Disable background-index on lit-tests by default

Summary:
Since background-index can perform disk writes, we don't want to turn
it on tests that won't clear it.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64990

llvm-svn: 366575