如果要测试服务器通过WebSocket连接对客户端请求的响应结果，可以使用ClientWebSocket类的ReceiveAsync方法从服务器接收响应，然后Assert该响应与预期值匹配。
您需要修改代码，如下所示：

[Fact]
public async Task WebSocket_Sending()
{
    
    var client = new ClientWebSocket();
    await client.ConnectAsync(Uri, CancellationToken.None);

    // Send the request to the server over the WebSocket connection.
    var sending = client.SendAsync(data, WebSocketMessageType.Binary, CancellationToken.None);

    // Receive the response from the server.
    var responseBuffer = new ArraySegment<byte>(new byte[1024]);
    var response = await client.ReceiveAsync(responseBuffer, CancellationToken.None);
    var responseString = Encoding.UTF8.GetString(responseBuffer, 0, response.Count);
    
    // Assert that the response is what you expect.
    Assert.AreEqual(responseString, "response");
}

您可能需要根据特定的使用情况调整缓冲区大小和编码类型。
或者，您可以使用模拟库或框架（如Moq）来模拟服务器对客户机请求的响应，并Assert客户机正在向模拟的服务器发送正确的请求数据。