[clangd] Switch FindSymbolsTests to use TestTU

Summary:
This gets rid of the dependency on ClangdServer and a bunch of extra
infrastructure coming with it. Also enables us to clear SyncAPI, as it was the
sole user of runWorkspace/DocumentSymbols.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82944