Nancy Testing with MS.Owin.Testing based on work from @damianh

AppRegistrations.cs

public class AppRegistrations
{
        private INancyBootstrapper bootstrapper;

        public INancyBootstrapper Bootstrapper
        { 
            get{ return bootstrapper ?? new MyBootstrapper(); }
            set{ bootstrapper = value; }
        }
}

mytest.cs

public class MyTest
{
        [Fact]
        public async Task Should_Return_200_On_Allowed_Path()
        {
            //Given
            var client = this.CreateHttpClient();
        
            //When
            var response = await client.PostAsync("http://localhost/api/myroute", new StringContent(string.Empty));
        
            //Then
            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
        }

        private HttpClient CreateHttpClient()
        {
            var setup = new OwinTestSetup();
            return setup.CreateHttpClient(with =>
                {
                    with.Module<SystemModule>();
                    with.Dependency<ISomethingCool>(A.Fake<ISomethingCool>());
                });
        }
}

setup.cs

public class OwinTestSetup
{
        public HttpClient CreateHttpClient(Action<ConfigurableBootstrapper.ConfigurableBootstrapperConfigurator> configureBootstrapperAction)
        {
            var appRegistrations = new AppRegistrations();

            appRegistrations.Bootstrapper = new ConfigurableBootstrapper(configureBootstrapperAction);

            var client = TestServer.Create(builder =>
                    new Startup(appRegistrations)
                    .Configuration(builder))
                .HttpClient;

            client.DefaultRequestHeaders.Add("Accept", "application/json");

            return client;
        }
}