Add example for flame_test (#958)

* Add example for flame_test

* Fix analyze warnings

* Fix tests

packages/flame_test/example/lib/main.dart

@@ -0,0 +1,15 @@
+import 'package:flame/extensions.dart';
+
+void main() {}
+
+class MyVectorChanger {
+  Vector2 addOne(Vector2 vector) {
+    return vector + Vector2.all(1.0);
+  }
+}
+
+class MyDoubleChanger {
+  double addOne(double number) {
+    return number + 1.0;
+  }
+}