chore: add GUT test framework

addons/gut/test/resources/replace_node_scenes/arena.gd

@@ -0,0 +1,13 @@
+extends Node2D
+
+func get_sword_ds():
+	return $Player1/Sword
+
+func get_sword():
+	return get_node('Player1/Sword')
+
+func get_player1_ds():
+	return $Player1
+
+func get_player1():
+	return get_node('Player1')