Add GUT (CLI only) and remove gd-plug

Commit GUT directly to the git repo and remove gd-plug as it is no
longer required to install GUT.

Modify GUT to be used by command-line only.

For example:
```
cp test/.gutconfig.ci.json .gutconfig.json
godot --no-window -s addons/gut/gut_cmdln.gd
```

addons/gut/hook_script.gd

@@ -0,0 +1,47 @@
+class_name GutHookScript
+# ------------------------------------------------------------------------------
+# This script is the base for custom scripts to be used in pre and post
+# run hooks.
+#
+# To use, inherit from this script and then implement the run method.
+# ------------------------------------------------------------------------------
+var JunitXmlExport = load("res://addons/gut/junit_xml_export.gd")
+
+# This is the instance of GUT that is running the tests.  You can get
+# information about the run from this object.  This is set by GUT when the
+# script is instantiated.
+var gut = null
+
+# the exit code to be used by gut_cmdln.  See set method.
+var _exit_code = null
+
+var _should_abort = false
+
+
+# Virtual method that will be called by GUT after instantiating
+# this script.
+func run():
+	gut.get_logger().error(
+		"Run method not overloaded.  Create a 'run()' method in your hook script to run your code."
+	)
+
+
+# Set the exit code when running from the command line.  If not set then the
+# default exit code will be returned (0 when no tests fail, 1 when any tests
+# fail).
+func set_exit_code(code):
+	_exit_code = code
+
+
+func get_exit_code():
+	return _exit_code
+
+
+# Usable by pre-run script to cause the run to end AFTER the run() method
+# finishes.  post-run script will not be ran.
+func abort():
+	_should_abort = true
+
+
+func should_abort():
+	return _should_abort