go/internal/xtesting: Retarget package

This package already provides parts of infrastructure for ZODB testing
(see e.g. 46636b16 and 7f14e2cb), and will be used as the place for such
infrastructure.

Maybe in the future we'll rename it to something like ztesting, but
leaving package name as is for now.

go/internal/xtesting/xtesting.go

-// Copyright (C) 2017-2019  Nexedi SA and Contributors.
+// Copyright (C) 2017-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -17,7 +17,7 @@
 // See COPYING file for full licensing terms.
 // See https://www.nexedi.com/licensing for rationale and options.
 
-// Package xtesting provides addons to std package testing.
+// Package xtesting provides infrastructure for ZODB testing.
 package xtesting
 
 import (