testing: use runtime/debug to format panics

Among other things, this avoids putting a testing.go:nnn:
prefix on every line of the stack trace.

R=golang-dev, r, dsymonds, r
CC=golang-dev
https://golang.org/cl/5651081

src/pkg/runtime/debug/stack_test.go

@@ -2,9 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package debug
+package debug_test
 
 import (
+	. "runtime/debug"
 	"strings"
 	"testing"
 )