container/heap: fix circular dependency in test

Having the test be in the container/heap package yields a cycle
  container/heap (for the test)
  -> testing
  -> time
  -> container/heap (for timerHeap)

Occasionally the linker would get mixed up, resulting in a test panic
in a very weird place.

R=rsc, r2
CC=golang-dev
https://golang.org/cl/4395042

src/pkg/container/heap/heap_test.go

@@ -2,11 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package heap
+package heap_test
 
 import (
 	"testing"
 	"container/vector"
+	. "container/heap"
 )