Method test_buffer

test/ostream-test.cc:137–139

  struct test_buffer final : fmt::detail::buffer<char> {
    explicit test_buffer(size_t size)
        : fmt::detail::buffer<char>([](buffer<char>&, size_t) {}, nullptr, size,
                                    size) {}
  } buffer(max_size);

  struct mock_streambuf : std::streambuf {