Integration Tests - Approach 1

1_TestStartup.cs

public class TestStartup : IStartup {
    private readonly string settings;

    public TestStartup(string settings) {
        this.settings = settings;
    }

    public void ConfigureServices(IServiceCollection services) {
       var configuration = new ConfigurationBuilder()
           .SetBasePath(Directory.GetCurrentDirectory())
           .AddJsonFile(settings, false) // Load appsettings.azure.json or appsettings.aws.json
           .AddEnvironmentVariables()
           .Build();

        services.AddMvc()
            .SetCompatibilityVersion(version: CompatibilityVersion.Version_2_2);

        //...Code to add required services based on configuration

    }

    public void Configure(IApplicationBuilder app) {
        app.UseMvc();

        //...Code to configure test Startup
    }
}

2_TestServerFixture.cs

public class TestServerFixture {
    static readonly Dictionary<string, TestServer> cache = 
        new Dictionary<string, TestServer>();

    public TestServerFixture() {
        //...
    }

    public HttpClient GetClient(string settings) {
        TestServer server = null;
        if(!cache.TryGetValue(settings, out server)) {
            var startup = new TestStartup(settings);
            var builder = new WebHostBuilder()
                .ConfigureServices(services => {
                    services.AddSingleton<IStartup>(startup);
                });
            server = new TestServer(builder);
            cache.Add(settings, server);
        }
        return server.CreateClient();
    }
}

3_MyTest.cs

public class MyTest : IClassFixture<TestServerFixture> {
    private readonly TestServerFixture fixture;

    public MyTest(TestServerFixture fixture) {
        this.fixture = fixture;
    }

    [Theory]
    [InlineData("appsettings.aws.json")]
    [InlineData("appsettings.azure.json")]
    public async Task ShouldValidateTheGetApiResponse(string settings) {
        var client = fixture.CreateClient(settings);

         var result = await client.GetAsync("SOME_API_URL");

        // Code to Assert Result

    }
}