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 d60d3455eb2b375d026a4aa74c4ba0c38f5d323c.