[clangd] Update TUStatus test to handle async PreambleThread

Summary:
Currently it doesn't matter because we run PreambleThread in sync mode.
Once we start running it in async, test order will become non-deterministic.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, mgrang, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77669