chore: add GUT test framework

addons/gut/test/unit/test_bugs/test_issue_SOTB.gd

@@ -0,0 +1,61 @@
+extends GutTest
+
+
+var SimpleScene = load("res://test/resources/simple_scene.tscn")
+var SimpleSceneScript = load("res://test/resources/simple_scene.gd")
+
+class TheSpawner:
+	@export var spawn_resource : SpawnResource
+
+	func spawn():
+		var inst = spawn_resource.scene.instantiate()
+		inst.foo()
+		return inst
+
+
+class SpawnResource:
+	extends Resource
+
+	# The bug is that if this has a type of PackedScene it would error when
+	# you tried to use a doubled scene for this value.
+	@export var scene : PackedScene = null
+
+	func foo():
+		return "bar"
+
+
+
+func before_all():
+	register_inner_classes(get_script())
+
+
+func test_are_we_sure_scenes_can_be_doubled():
+	var DoubleSimpleScene = double(SimpleScene)
+	var inst = DoubleSimpleScene.instantiate()
+
+	assert_not_null(inst)
+	assert_has_method(inst, "foo")
+
+
+func test_can_use_a_double_of_a_scene_as_the_value_of_a_PackedScene_variable():
+	var DoubleSimpleScene = double(SimpleScene)
+	var res = SpawnResource.new()
+	res.scene = DoubleSimpleScene
+
+	var spawner = TheSpawner.new()
+	spawner.spawn_resource = res
+
+	var spawned = spawner.spawn()
+	assert_not_null(spawned)
+
+
+func test_can_use_a_double_of_a_scene_as_the_value_of_a_PackedScene_variable2():
+	var DoubleSimpleScene = double(SimpleScene)
+	var res = SpawnResource.new()
+	res.scene = DoubleSimpleScene
+
+	var spawner = TheSpawner.new()
+	spawner.spawn_resource = res
+
+	var spawned = spawner.spawn()
+	assert_ne(spawned.this_is_different_in_scene, "default")