[clangd] More unittests for cross-file rename.

Summary:
The previous unittests for cross-file rename was kind of weak. With this
patch, we should have more test coverage, and it is easy to add more tests in
the future.

Reviewers: ilya-biryukov, kbobyrev

Reviewed By: ilya-biryukov

Subscribers: merge_guards_bot, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71050