chore: add GUT test framework

addons/gut/test/samples/test_output_demo.gd

@@ -0,0 +1,25 @@
+extends 'res://addons/gut/test.gd'
+
+func test_passing():
+	assert_eq('one', 'one')
+
+func test_failing():
+	assert_eq(1, 'two')
+
+func test_pending_no_text():
+	pending()
+
+func test_pending_with_text():
+	pending('this has text')
+
+func test_parameterized_passing(p=use_parameters([1, 2, 3, 4])):
+	assert_gt(p, 0)
+
+func test_parameterized_failing(p = use_parameters([[1, 2], [3, 4]])):
+	assert_eq(p[0], p[1])
+
+func test_assert_called_failing():
+	var d = double(Node2D).new()
+	d.set_position(Vector2(1, 1))
+	d.get_name()
+	assert_called(d, 'get_position')