message: Fix include order for concurrent append test to please windows
diff: #include "catch.hpp"
  -#include "main/appender.hpp"
  -#include "test_helpers.hpp"
  #include <thread>
  #include <vector>
  +// needs to happen after the <thread> include since we redefine make_unique
  +#include "main/appender.hpp"
  +#include "test_helpers.hpp"
  +
  using namespace duckdb;
  using namespace std;
  