chore: add GUT test framework

addons/gut/test/resources/doubler_test_objects/inner_classes.gd

@@ -0,0 +1,44 @@
+class_name GutDoubleTestInnerClasses
+
+var _value = 1
+
+
+func get_value():
+	return _value
+
+func set_value(val):
+	_value = val
+
+
+class InnerA:
+	func get_a():
+		return 'a'
+
+
+# Needed another class with same method as an inner class to test
+# stubbing.
+class AnotherInnerA:
+	func get_a():
+		return 'aia'
+
+class InnerB:
+	func get_b():
+		return 'b'
+
+	class InnerB1:
+		func get_b1():
+			return 'b1'
+
+class InnerCA:
+	extends InnerA
+
+	func get_ca():
+		return 'ca'
+
+class InnerWithSignals:
+	signal signal_signal
+
+
+
+class InnerExtendsNode2D:
+	extends Node2D