test(app): add app test scaffolding and headless test harness

tests/Vantage.App.Tests/HeadlessTestApp.cs

@@ -0,0 +1,15 @@
+using Avalonia;
+using Avalonia.Headless;
+using Vantage.App;
+using Vantage.App.Tests;
+
+[assembly: AvaloniaTestApplication(typeof(HeadlessTestApp))]
+
+namespace Vantage.App.Tests;
+
+public static class HeadlessTestApp
+{
+    public static AppBuilder BuildAvaloniaApp() => AppBuilder
+        .Configure<App>()
+        .UseHeadless(new AvaloniaHeadlessPlatformOptions());
+}