[tests] add test for #9571 (#11974)

tests/misc/projects/Issue9571/Main.hx

@@ -0,0 +1,13 @@
+class Main {
+	static function main() {}
+
+	static function f<TA:A>(v:TA) {
+		v.defined(); // works
+		v.forwarded(); // doesn't work before 4.2
+	}
+}
+
+@:forward(forwarded)
+abstract A({function forwarded():Void;}) {
+	public function defined() {}
+}

tests/misc/projects/Issue9571/compile.hxml

@@ -0,0 +1 @@
+-main Main