simple_shader: Clean up shaders/simple.frag, rebuild runners (#1715)

simple_shader/test/widget_test.dart

@@ -0,0 +1,9 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:simple_shader/main.dart';
+
+void main() {
+  testWidgets('Smoke test', (tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(const MyApp());
+  });
+}