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/thing_counter.gd

@@ -0,0 +1,51 @@
+var things = {}
+
+
+func get_unique_count():
+	return things.size()
+
+
+func add(thing):
+	if things.has(thing):
+		things[thing] += 1
+	else:
+		things[thing] = 1
+
+
+func has(thing):
+	return things.has(thing)
+
+
+func get(thing):
+	var to_return = 0
+	if things.has(thing):
+		to_return = things[thing]
+	return to_return
+
+
+func sum():
+	var count = 0
+	for key in things:
+		count += things[key]
+	return count
+
+
+func to_s():
+	var to_return = ""
+	for key in things:
+		to_return += str(key, ":  ", things[key], "\n")
+	to_return += str("sum: ", sum())
+	return to_return
+
+
+func get_max_count():
+	var max_val = null
+	for key in things:
+		if max_val == null or things[key] > max_val:
+			max_val = things[key]
+	return max_val
+
+
+func add_array_items(array):
+	for i in range(array.size()):
+		add(array[i])