[clangd] Drop TestTUs dependency on gtest

TestTU now prints errors to llvm::errs and aborts on failures via
llvm_unreachable, rather than executing ASSERT_FALSE.

We'd like to make use of these testing libraries in different test suits that
might be compiling with a different gtest version than LLVM has.

Differential Revision: https://reviews.llvm.org/D103685