Improve windows dotnet firefox perf (#129)

#### Description of changes

During local testing of #128, I noticed that the .NET Core + Windows + Firefox environment combination was excessively slow compared to all other combinations (taking >2min per test run instead of <10s).

This turns out to be caused by mozilla/geckodriver#1496 / https://bugzilla.mozilla.org/show_bug.cgi?id=1525659 - if I'm understanding the bugs correctly, geckodriver listens for selenium commands on IPv4 only by default and selenium tries to connect via "http://localhost" rather than "http://127.0.0.1", but .NET Core's standard HttpClient behavior under Windows ends up attempting to connect to "localhost" URLs via IPv6 for a second before timing out and retrying on IPv4, so in practice this adds a second of overhead to every API call between Selenium and geckodriver.

This change works around it by forcing both of them to use a `::1` to communicate. The linked bug suggested that using `localhost` or `127.0.0.1` should have also worked around the issue, but it didn't seem to help when I tried it. Using `::1` in all cases prevented it from working in our linux build agents, so I ended up having to do a platform check.

Before:
```
> dotnet test --filter TestCategory!=IntentionallyFailsAsAnExample
Test run for Q:\repos\axe-pipelines-samples\csharp-selenium-webdriver-sample\bin\Debug\netcoreapp2.2\CSharpSeleniumWebdriverSample.dll(.NETCoreApp,Version=v2.2)
Microsoft (R) Test Execution Command Line Tool Version 16.0.1
Copyright (c) Microsoft Corporation.  All rights reserved.
Starting test execution, please wait...
Total tests: 3. Passed: 3. Failed: 0. Skipped: 0.
Test Run Successful.
Test execution time: 2.3717 Minutes
Command took 2:23.929.
```

After:
```
> dotnet test --filter TestCategory!=IntentionallyFailsAsAnExample
Test run for Q:\repos\axe-pipelines-samples\csharp-selenium-webdriver-sample\bin\Debug\netcoreapp2.2\CSharpSeleniumWebdriverSample.dll(.NETCoreApp,Version=v2.2)
Copyright (c) Microsoft Corporation.  All rights reserved.
Starting test execution, please wait...
Total tests: 3. Passed: 3. Failed: 0. Skipped: 0.
Test Run Successful.
Test execution time: 5.4795 Seconds
Command took 7.2035856 seconds.
```

#### Pull request checklist

- [n/a] If this PR addresses an existing issue, it is linked: Fixes #0000
- [x] New sample content is commented at a similar verbosity as existing content
- [x] All updated/modified sample code builds and runs in at least one PR/CI build
- [x] PR checks for builds named `[failing example] ...` fail as expected

csharp-selenium-webdriver-sample/WebDriverFactory.cs

@@ -54,7 +54,13 @@ public static IWebDriver CreateFromEnvironmentVariableSettings() {
                     var firefoxOptions = new FirefoxOptions();
                     firefoxOptions.AddArgument("--headless");
 
-                    return new FirefoxDriver(geckoDriverDirectory, firefoxOptions);
+                    var firefoxDriverService = FirefoxDriverService.CreateDefaultService(geckoDriverDirectory);
+                    // This is a workaround for Windows-specific performance issues caused by https://github.com/mozilla/geckodriver/issues/1496
+                    if (Platform.CurrentPlatform.IsPlatformType(PlatformType.Windows)) {
+                        firefoxDriverService.Host = "::1";
+                    }
+
+                    return new FirefoxDriver(firefoxDriverService, firefoxOptions);
 
                 default:
                     throw new ArgumentException($"Unknown browser type '{browserEnvVar}' specified in '{BROWSER_ENVIRONMENT_VARIABLE}' environment variable");