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/compare_result.gd

@@ -0,0 +1,60 @@
+var are_equal = null setget set_are_equal, get_are_equal
+var summary = null setget set_summary, get_summary
+var max_differences = 30 setget set_max_differences, get_max_differences
+var differences = {} setget set_differences, get_differences
+
+
+func _block_set(which, val):
+	push_error(str("cannot set ", which, ", value [", val, "] ignored."))
+
+
+func _to_string():
+	return str(get_summary())  # could be null, gotta str it.
+
+
+func get_are_equal():
+	return are_equal
+
+
+func set_are_equal(r_eq):
+	are_equal = r_eq
+
+
+func get_summary():
+	return summary
+
+
+func set_summary(smry):
+	summary = smry
+
+
+func get_total_count():
+	pass
+
+
+func get_different_count():
+	pass
+
+
+func get_short_summary():
+	return summary
+
+
+func get_max_differences():
+	return max_differences
+
+
+func set_max_differences(max_diff):
+	max_differences = max_diff
+
+
+func get_differences():
+	return differences
+
+
+func set_differences(diffs):
+	_block_set("differences", diffs)
+
+
+func get_brackets():
+	return null