No connection could be made because the target machine actively refused it. (localhost:80) when attempting to test routes with TestServer

Question

I'm attempting to use Microsoft.AspNetCore.TestHost to test routes via TestServer. I believe the WebHost setup I have should be sufficient; however, whenever I run my test, I receive an error message.

Test method ID_Test.Controllers.Test_CloudPrintController_RouteTests.AppController_ShouldRouteCorrectly threw exception: System.Net.Http.HttpRequestException: No connection could be made because the target machine actively refused it. (localhost:80) ---> System.Net.Sockets.SocketException: No connection could be made because the target machine actively refused it.

From my understanding, this is either related to firewall issues or incorrect appsettings; but if my appsettings work for all my other non-TestServer tests, and this uses the same Program.cs / Startup.cs, then I feel as if it should work for this as well.

Here is my test Class and the Mock Services I made.

public class MockAuthenticationService
{
    public async Task SignInAsync(IServiceProvider serviceProvider)
    {
        // Mock implementation of signing in
        // Contents removed for brevity
    }
}

public class MockAuthenticationSchemeProvider : IAuthenticationSchemeProvider
{
    // Contents removed for brevity
}

public class MockAuthenticationHandler : IAuthenticationHandler
{
    // Contents removed for brevity
}

public class MockAuthenticationHandlerProvider : IAuthenticationHandlerProvider
{
    // Contents removed for brevity
}

public class MockClaimsTransformation : IClaimsTransformation
{
    // Contents removed for brevity
}

public class MockOptionsAuthentication : IOptions<AuthenticationOptions>
{
    // Contents removed for brevity
}

[TestClass]
public class Test_CloudPrintController_RouteTests : WebApplicationFactory<Program>
{
    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureTestServices(services =>
        {
            // Register mock implementations
            services.AddSingleton<IAuthenticationSchemeProvider, MockAuthenticationSchemeProvider>();
            services.AddSingleton<IAuthenticationHandlerProvider, MockAuthenticationHandlerProvider>();
            services.AddSingleton<IClaimsTransformation, MockClaimsTransformation>();
            services.AddSingleton<IOptions<AuthenticationOptions>, MockOptionsAuthentication>();
            services.AddSingleton<MockAuthenticationService>();

            // Register AuthenticationService with correct dependencies
            services.AddSingleton(sp =>
            {
                var mockAuthService = sp.GetRequiredService<MockAuthenticationService>();
                var mockSchemeProvider = sp.GetRequiredService<IAuthenticationSchemeProvider>();
                var mockHandlers = sp.GetRequiredService<IAuthenticationHandlerProvider>();
                var mockTransform = sp.GetRequiredService<IClaimsTransformation>();
                var mockOptions = sp.GetRequiredService<IOptions<AuthenticationOptions>>();

                return new AuthenticationService(mockSchemeProvider, mockHandlers, mockTransform, mockOptions);
            });

            // Add JWT authentication for testing purposes
            services.AddAuthentication(options =>
            {
                options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
            }).AddJwtBearer(options =>
            {
                options.RequireHttpsMetadata = false; // In test environment, set to false for simplicity
                options.TokenValidationParameters = new TokenValidationParameters
                {
                    ValidateIssuer = false, // Set to false for testing
                    ValidateAudience = false, // Set to false for testing
                    ValidateLifetime = false, // Set to false for testing
                    ValidateIssuerSigningKey = false, // Set to false for testing
                                                      // Modify other parameters as needed for testing purposes
                };
            });
        });
    }

    [TestMethod]
    [DoNotParallelize]
    public async Task AppController_ShouldRouteCorrectly()
    {
        // Arrange
        HttpClient _client = CreateClient();

        var cookieContainer = new CookieContainer();
        cookieContainer.Add(_client.BaseAddress, new Cookie("Cookie", "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.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.MalKNawXYB0fdfe1644Ut_4rjb1W2QaBw236WVJrHTM"));
        var handler = new HttpClientHandler { CookieContainer = cookieContainer };
        _client = new HttpClient(handler) { BaseAddress = _client.BaseAddress };

        // Simulate authentication by adding mock authentication token/header
        _client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.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.MalKNawXYB0fdfe1644Ut_4rjb1W2QaBw236WVJrHTM");

        // Act
        var devResponse = await _client.GetAsync("/developer"); // 100 is the test printer node
        var response = await _client.GetAsync("/api/subscription-bundles"); // 100 is the test printer node

        // Assert
        Assert.AreEqual(HttpStatusCode.OK, response.StatusCode); // This 404's, presumably cannot reach this endpoint through the test code
    }
}

Thanks in advance for your help!