Tidyup varfont_shader_puzzle (#1629)

* Tidyup `varfont_shader_puzzle`

* Add a rebuild script and a test

* Simplify fragment shaders

* Drop web

* Enable `beta` CI

* make it `beta` inclusive

* Pull out the program loading code

experimental/varfont_shader_puzzle/test/widget_test.dart

@@ -0,0 +1,26 @@
+// Copyright 2023 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:varfont_shader_puzzle/main.dart';
+
+void main() {
+  const welcomeText =
+      'Welcome to your first day on the FontCo team! Are you ready to help us publish our newest font, Designer Pro?';
+  const welcomeTextStep2 =
+      'Oh no, you clicked the button too hard! Now the font file is glitched. Help us put the letters back together so we can launch!';
+
+  testWidgets('Initial display', (tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(const TypePuzzle());
+
+    // Verify intro text
+    expect(find.text(welcomeText), findsOneWidget);
+    expect(find.text(welcomeTextStep2), findsNothing);
+
+    // Verify OK button
+    expect(find.text('OK'), findsOneWidget);
+  });
+}