问为什么TestServer (AspNetCore)在静态文件上出现404错误？

EN

var host = new WebHostBuilder()
                .UseKestrel()
                .UseContentRoot(Directory.GetCurrentDirectory())
                .UseIISIntegration()
                .UseStartup<Startup>()
                .Build();

            host.Run();

public void Configure(IApplicationBuilder app)
{
        app.Run(async (context) =>
        {
            await context.Response.WriteAsync("Hello World!");
        });
}

        public TestFixture()
        {
            var builder = new WebHostBuilder().UseStartup<TStartup>();
            _server = new TestServer(builder);

            Client = _server.CreateClient();
            Client.BaseAddress = new Uri(address);
        }

        var response = await Client.GetAsync("http://localhost:51021/");
        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
        var responseString = await response.Content.ReadAsStringAsync();
        var content = await response.Content.ReadAsStringAsync();
        Assert.Contains("Hello World!", content);

    public void Configure(IApplicationBuilder app)
    {
        app.UseDefaultFiles();
        app.UseStaticFiles();
        //app.Run(async (context) =>
        //{
        //    await context.Response.WriteAsync("Hello World!");
        //});
    }