How to properly configure mock dependencies for testing Web Api (ASP.NET Core) controllers using Autofac

I'm using ASP.NET Core (2.0) with Autofac, and a Microsoft.AspNetCore.TestHost.TestServer for integration testing. However, for some test scenarios, I would like to inject some service mocks instead of the implementations loaded in ConfigureContainer method (as described here: http://ift.tt/2xvju4M).

Example:

public class Program
{
    public static void Main(string[] args)
    {
        var host = new WebHostBuilder()
            .UseKestrel()
            .ConfigureServices(s => s.AddAutofac())
            .UseStartup<Startup>()
            .Build();

        host.Run();
    }
}

public class Startup
{
    ...
    public void ConfigureContainer(ContainerBuilder builder)
    {
        builder.RegisterModule(new Modules.ApiModule());
    }
    ...
}

And the test class:

public class BasicControllerTests
{
    TestServer server;
    HttpClient client;

    public BasicControllerTests()
    {
        var resellerRepo = new Mock<IResellerProvider>();
        resellerRepo.Setup(a => a.Query())
            .Returns(new[] {
            new Model.Reseller
            {
                Id = Guid.NewGuid(),
                Code = "R1",
                Name = "Reseller 1"
            }
            }.AsQueryable());

        // How to inject mock properly in the lines below?

        server = new TestServer(new WebHostBuilder()
            .ConfigureServices(a => a.AddAutofac())
            .UseStartup<Startup>());

        client = server.CreateClient();
    }
   ...

What I would like to do is to use the TestServer with all the dependencies as they are, but just the IResellerProvider mocked as in the test example. What is the best way to accomplish that? Of course, I could create a TestStartup class for this exact case, but I would like to know what is the proper way to handle this situation.