Fix gcc compilation warning in test case [NFC]

Without this, gcc (7.4) complains with

../tools/clang/tools/extra/clangd/unittests/PrintASTTests.cpp:99:28: error: ISO C++11 requires at least one argument for the "..." in a variadic macro [-Werror]
                         })));
                            ^

llvm-svn: 360334