Add emission_finished signal to Particles2D

(Also remove some Particles2D::testee method, which was unused)
Resolves #3963

scene/scene_string_names.h

@@ -80,6 +80,7 @@ public:
 	StringName sort_children;
 
 	StringName finished;
+	StringName emission_finished;
 	StringName animation_finished;
 	StringName animation_changed;
 	StringName animation_started;