[clang-tools-extra] Fix linking ClangdTests when using libclang-cpp

Fix linking ClangdTests to specify the dependency on the private
clangTesting library via target_link_libraries() rather than
clang_target_link_libraries().  The latter uses libclang-cpp when
CLANG_LINK_CLANG_DYLIB is used, and clangTesting is not included
in this library.

This fixes d60d3455.

Differential Revision: https://reviews.llvm.org/D146427