chore: add GUT test framework

addons/gut/documentation/docs/Inner-Test-Classes.md

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+# Inner Test Classes
+
+You can define test classes inside a test script that will be treated as test scripts themselves.  This allows you to create different contexts for your tests in a single script.  These Inner Classes have their own `before_all`, `before_each`, `after_each`, and `after_all` methods that will be called.  Only the methods defined in the class are used, the methods defined in the containing script will not be called.
+
+The Inner Classes must also extend `GutTest` and their constructor cannot take any parameters.  The Classes will be loaded and ran in the order they are defined _after_ all the tests in the containing script are run.  If the script does not contain any tests then only the Inner Classes will be listed in the output.
+
+The order the tests are run are not guaranteed to be in the same order they are defined (I don't know why yet).
+
+Inner Classes are parsed out of the script by looking for a classes that start with `'Test'` and also extend `test.gd`.  You can change the name that Gut looks for using the `inner_class_prefix` property.
+
+## Example
+Given the following test script defined at `res://test/unit/some_example.gd`
+```
+extends GutTest
+
+func before_all():
+	gut.p('script:  pre-run')
+
+func before_each():
+	gut.p('script:  setup')
+
+func after_each():
+	gut.p('script:  teardown')
+
+func after_all():
+	gut.p('script:  post-run')
+
+func test_something():
+	assert_true(true)
+
+class TestClass1:
+	extends GutTest
+
+	func before_all():
+		gut.p('TestClass1:  pre-run')
+
+	func before_each():
+		gut.p('TestClass1:  setup')
+
+	func after_each():
+		gut.p('TestClass1:  teardown')
+
+	func after_all():
+		gut.p('TestClass1:  post-run')
+
+	func test_context1_one():
+		assert_true(true)
+
+	func test_context1_two():
+		pending()
+```
+
+Gut will generate this following when running the test script.
+
+```
+/-----------------------------------------
+Running Script res://test/unit/some_sample.gd
+-----------------------------------------/
+script:  pre-run
+* test_something
+    script:  setup
+    PASSED:
+    script:  teardown
+
+/-----------------------------------------
+Running Class [TestClass1] in res://test/unit/some_sample.gd
+-----------------------------------------/
+TestClass1:  pre-run
+* test_context1_two
+    TestClass1:  setup
+    Pending
+    TestClass1:  teardown
+* test_context1_one
+    TestClass1:  setup
+    PASSED:
+    TestClass1:  teardown
+    TestClass1:  post-run
+```