GUT测试中调用KinematicBody2D的get_position报get_global_transform错误

我尝试使用GUT为GDScript中的player节点编写单元测试，该节点属于KinematicBody2D类型，调用其get_position()方法时触发如下错误：

ERROR: get_global_transform: Condition "!is_inside_tree()" is true. Returned: get_transform()
   At: scene/2d/canvas_item.cpp:467.
ERROR: body_test_motion: Condition "!body->get_space()" is true. Returned: false
   At: servers/physics_2d/physics_2d_server_sw.cpp:1046.
ERROR: get_global_transform: Condition "!is_inside_tree()" is true. Returned: get_transform()
   At: scene/2d/canvas_item.cpp:467.
ERROR: body_test_ray_separation: Condition "!body->get_space()" is true. Returned: false
   At: servers/physics_2d/physics_2d_server_sw.cpp:1058.

预期表现 #

我预期在_physics_process()执行后可以获取到player更新后的position，但实际返回值为Vector2(0, 0)。

我是首次使用GDScript，编写该测试时参考了以下资料：

• 8-way movement
• TDD and PONG Episode 1
• GUT Command-line
• Stubbing Input Class

已尝试的无效解决方案 #

我猜测需要将player挂载为其他节点的子节点，但不清楚如何在测试中实现该操作。
我检索了Gut issues中关于测试内使用add_child()的内容，未找到对应解决方案。
我还尝试将player挂载为Area2D的子节点，也没有生效，代码如下：

var player := Player.new(input)
var area := Area2D.new()
area.add_child(player)

以下是命令行调用gut时触发错误的对应代码：

test/unit/actors/test_player.gd

extends "res://addons/gut/test.gd"

func test_can_create_player() -> void:
    var input = MockInput.new()
    var player := Player.new(input)
    
    assert_not_null(player)

func test_can_move_player_up() -> void:
    var input = MockInput.new()
    input.press("ui_up")

    var player := Player.new(input)

    simulate(player, 1, .1)
    assert_eq(player.get_position(), Vector2(200, 0))

test/mock_input.gd

class_name MockInput

var _pressed: Dictionary = {}

func press(key: String) -> void:
    _pressed[key] = true

func release(key: String) -> void:
    _pressed[key] = false

func is_action_pressed(key: String) -> bool:
    if _pressed.has(key):
        return _pressed.get(key)

    return false

entities/actors/player.gd

extends KinematicBody2D

class_name Player

export var speed: int = 200

var _input: Object = Input
var _velocity: Vector2 = Vector2()

func _init(input: Object = Input):
    _input = input

func _physics_process(_delta: float) -> void:
    _velocity = Vector2()
    
    if _input.is_action_pressed("ui_right"):
        _velocity.x += 1
    if _input.is_action_pressed("ui_left"):
        _velocity.x -= 1
    if _input.is_action_pressed("ui_down"):
        _velocity.y += 1
    if _input.is_action_pressed("ui_up"):
        _velocity.y -= 1

    _velocity = _velocity.normalized() * speed
    _velocity = move_and_slide(_velocity)

内容的提问来源于stack exchange，提问作者SpeedyLom