[clangd] Boost completion score according to file proximity.

Summary:
Also move unittest: URI scheme to TestFS so that it can be shared by
different tests.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D47935

llvm-svn: 334810