message: Fix udp_datagram_socket test
diff: @@ -77,13 +77,17 @@ struct fixture : host_fixture {
  else
  CAF_FAIL("write returned an error: " << sys.render(get<sec>(write_ret)));
  - auto read_ret = read(receive_socket, make_span(buf));
  + variant<std::pair<size_t, ip_endpoint>, sec> read_ret;
  + do {
  + read_ret = read(receive_socket, make_span(buf));
  if (auto read_res = get_if<std::pair<size_t, ip_endpoint>>(&read_ret)) {
  CAF_CHECK_EQUAL(read_res->first, hello_test.size());
  buf.resize(read_res->first);
  - } else {
  - CAF_FAIL("write returned an error: " << sys.render(get<sec>(read_ret)));
  + } else if (get<sec>(read_ret) != sec::unavailable_or_would_block) {
  + CAF_FAIL("read returned an error: " << sys.render(get<sec>(read_ret)));
  }
  + } while (holds_alternative<sec>(read_ret)
  + && get<sec>(read_ret) == sec::unavailable_or_would_block);
  string_view buf_view{reinterpret_cast<const char*>(buf.data()), buf.size()};
  CAF_CHECK_EQUAL(buf_view, hello_test);
  }
  