Object: Add tests about the safety of tail destruction

(cherry picked from commit bb77520599)

modules/gdscript/tests/scripts/runtime/features/self_destruction.gd

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+# https://github.com/godotengine/godot/issues/75658
+
+class MyObj:
+	var callable: Callable
+
+	func run():
+		callable.call()
+
+	var prop:
+		set(value):
+			callable.call()
+		get:
+			callable.call()
+			return 0
+
+	func _on_some_signal():
+		callable.call()
+
+	func _init(p_callable: Callable):
+		self.callable = p_callable
+
+signal some_signal
+
+var obj: MyObj
+
+func test():
+	# Call.
+	obj = MyObj.new(nullify_obj)
+	obj.run()
+	print(obj)
+
+	# Get.
+	obj = MyObj.new(nullify_obj)
+	var _aux = obj.prop
+	print(obj)
+
+	# Set.
+	obj = MyObj.new(nullify_obj)
+	obj.prop = 1
+	print(obj)
+
+	# Signal handling.
+	obj = MyObj.new(nullify_obj)
+	@warning_ignore("return_value_discarded")
+	some_signal.connect(obj._on_some_signal)
+	some_signal.emit()
+	print(obj)
+
+func nullify_obj():
+	obj = null

modules/gdscript/tests/scripts/runtime/features/self_destruction.out

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+GDTEST_OK
+<null>
+<null>
+<null>
+<null>