[clangd] Fix raciness in code completion tests Reviewers: sammccall, ilya-biryukov Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits Tags: #clang Differential Revision: https://reviews.llvm.org/D68273 git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@373924 91177308-0d34-0410-b5e6-96231b3b80d8