public class RestaurantApiFactory : WebApplicationFactory<Startup>
{
    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        if (builder  is null)
            throw new ArgumentNullException(nameof(builder));

        builder.ConfigureServices(services =>
        {
            services.RemoveAll<IReservationsRepository>();
            services.AddSingleton<IReservationsRepository>(
                new FakeDatabase());
        });
    }
}