How to fake declared services in Startup.cs during testing?

I would like to write integration tests for my Asp .net core application, but I don't want my tests to use real implemetation of some services.

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        ...
        services.AddTransient<IExternalService,ExternalService>();
        ...
    }
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        ...
    }
}

public interface IExternalService
{
    bool Verify(int id);
}

public class ExternalService : IExternalService
{
    public bool Verify(int id)
    {
        //Implemetation is here.
        //I want to fake this implemetation during testing.
    }
}

[Fact]
public void TestCase()
{
    //Stub out service
    var myExtService = new Mock<IExternalService>();

    //Setup response by stub
    myExtService
        .Setup(p => p.Verify(It.IsAny<int>()))
        .Returns(false);

    var host = new WebHostBuilder()
        .UseStartup<Startup>()
        .ConfigureServices((services) =>
        {
            //Setup injection
            services.AddTransient<IExternalService>((a) =>
            {
                return myExtService.Object;
            });
        });

    var server = new TestServer(host);

    var client = server.CreateClient();

    var response = client.GetAsync("/").Result;

    var responseString = response.Content.ReadAsStringAsync().Result;

    Assert.Contains("Your service returned: False", responseString);
}

Current injection setup in test case does not work, because ExternalService is injected over the mock.

However the test will pass when I remove services.AddTransient<IExternalService,ExternalService>; from Startup.

Most likely the one in Startup is called later and all the setup in that class is preferred by application.

What options do I have to setup some dependecies in tests, but use everything else as they are declared in Startup?