找到了一个好东东：Moq.Contrib.HttpClient

在测试中 mock HttpClient 的试验代码如下

[Theory]
[InlineData(1, 2, 3)]
public async Task GetPublicPostIds_test(params int[] postIds)
{
    var publicPostIds = postIds.ToList();
    publicPostIds.Remove(Random.Shared.Next(1, 3));

    var handler = new Mock<HttpMessageHandler>();
    var httpClient = handler.CreateClient();
    httpClient.BaseAddress = new Uri("http://blog_api");
    handler.SetupRequest(HttpMethod.Post, "http://blog_api/v2/blogposts/public/ids")
        .ReturnsJsonResponse(publicPostIds);

    var actualPostIds = await new BlogClient(httpClient, _logger).BlogPosts
        .GetPublicPostIds(postIds);
    Assert.NotEmpty(actualPostIds);
    actualPostIds.Should().BeEquivalentTo(publicPostIds);
}