[clangd] Prevent thread starvation in tests on loaded systems.

Summary:
Background index deliberately runs low-priority, but for tests this may stop
them making progress.

Reviewers: kadircet

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

Differential Revision: https://reviews.llvm.org/D54938

llvm-svn: 347655