[clangd] Split code-completion tests out of ClangdTests. NFC.

Summary:
Common parts are mostly FS related, so pulled out TestFS.h for the common stuff.
Deliberately resisted cleaning up much here, so this is pretty mechanical.

Reviewers: hokein

Subscribers: klimek, mgorny, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D40784

llvm-svn: 319741