testing/quick, text/tabwriter: freeze packages

Fixes #15557

Change-Id: I02ad98068894e75d4e08e271fdd16cb420519460
Reviewed-on: https://go-review.googlesource.com/31910Reviewed-by: Andrew Gerrand <adg@golang.org>

src/testing/quick/quick.go

@@ -3,6 +3,8 @@
 // license that can be found in the LICENSE file.
 
 // Package quick implements utility functions to help with black box testing.
+//
+// The package is frozen and not accepting new features.
 package quick
 
 import (

src/text/tabwriter/tabwriter.go

@@ -8,6 +8,7 @@
 // The package is using the Elastic Tabstops algorithm described at
 // http://nickgravgaard.com/elastictabstops/index.html.
 //
+// The package is frozen and not accepting new features.
 package tabwriter
 
 import (