Service is not using DbContext when using TestServer

In Startup I isolated ConfigureAppServices to a separate private method so I can override it in TestStartup in my Test project

public virtual void ConfigureMyServices(IServiceCollection services)
{
    services.AddScoped<ICarService, CarService>();
}

same thing for database

public virtual void ConfigureDatabase(IServiceCollection services)
{
    services.AddDbContext<CarContext>(options =>
    options.UseSqlServer(Configuration.GetConnectionString("CarDbConnection")));
}

public class TestStartup : Startup
{
    private readonly Mock<ICarService> _carServiceMock;
    public TestStartup(IConfiguration configuration) : base(configuration)
    {
        _carServiceMock = new Mock<ICarService>();
    }

    public override void ConfigureMyServices(IServiceCollection services)
    {
        services.AddSingleton(_carServiceMock.Object);
    }

    public override void ConfigureDatabase(IServiceCollection services)
    {
        services.AddDbContext<CarContext>(options => options.UseInMemoryDatabase("CarDb"));
        services.AddTransient<DataSeed>();
    }   
}

TestFixture is using TestStartup

public TestFixture()
{
     _server = new TestServer(new WebHostBuilder()
          .UseStartup<TestStartup>()
          .UseEnvironment("Development"));

     Client = _server.CreateClient();
}

Inside my controller I'm using ICarService to fetch data. ICarService implementation using CarContext.

public class CarController
{
    private readonly ICarService _carService;

    public CarController(ICarService carService)
    {
        _carService = carService;            
    }

    [HttpGet]
    public async Task<IActionResult> Get([FromRoute] int id)
    {
        var contact = await _carService.GetAsync(id); // this is always null
    }
}

Am I injecting DbContext properly? Am I missing something? How can I be sure that CarContext is properly set and used by mocked ICarService?