chore: add GUT test framework

addons/gut/documentation/docs/Partial-Doubles.md

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+# Partial Doubles
+
+A Partial Double is the same thing as a normal Double except all methods retain their functionality by default.
+
+You can create Partial Doubles for scripts, packed scenes, and inner classes.  It works the same way as `double` works, so read up on that and then apply all that new knowledge to `partial_double`.
+
+Under the covers, whether you call `double` or `partial_double`, GUT makes the same thing.  If you call `partial_double` though, it will setup the object's methods to be stubbed `to_call_super` instead of not being stubbed at all.
+
+After you have your partial double, you can stub methods to return values instead of doing what they normally do.  You can also spy on any of the methods.
+
+__NOTE__ All Doubles and Partial Doubles are freed when a test finishes.  This means you do not have to free them manually and you should not be created in `before_all` or referenced in `after_all`.
+
+## Script Example
+
+Given
+``` gdscript
+# res://foo.gd
+extends Node2D
+
+var _value = 10
+
+func set_value(val):
+  _value = val
+
+func get_value():
+  return _value
+```
+
+Then
+
+```gdscript
+var Foo = load('res://script.gd')
+
+func test_things():
+  var partial = partial_double(Foo).new()
+  stub(partial, 'set_value').to_do_nothing()
+  partial.set_value(20) # stubbed so implementation bypassed.
+
+  # since set_value was stubbed, and get_value was not, and since
+  # this is a partial stub, then the original functionality of
+  # get_value will be executed and _value is returned.
+  assert_eq(partial.get_value(), 10)
+  # unstubbed partial methods can be spied on.
+  assert_called(partial, 'get_value')
+  # stubbed methods can be spied on as well
+  assert_called(partial, 'set_value', [20])
+```