chore: add GUT test framework

addons/gut/test/output_tests/test_bugs.gd

@@ -0,0 +1,16 @@
+extends GutOutputTest
+
+# Issue 725.
+# The following error was created when Tweens were sent off to assert_is.
+"""
+E 0:00:04:930   utils.gd:434 @ get_native_class_name(): Tween can't be created directly. Use create_tween() method.
+  <C++ Error>   Method/function failed.
+  <C++ Source>  scene/animation/tween.cpp:537 @ Tween()
+  <Stack Trace> utils.gd:434 @ get_native_class_name()
+                strutils.gd:120 @ type2str()
+                test.gd:121 @ _str()
+                test.gd:1681 @ assert_is()
+"""
+func test_issue_725() -> void:
+	assert_is(create_tween(), Tween, 'expected valid tween')
+	should_not_error()