container/heap: correct number of elements in BenchmarkDup

In BenchmarkDup fuction, heap is created as h := make(myHeap, n)
and then n elements are added, so first time there are 2*n elements
in heap.

Fixes #15380

Change-Id: I0508486a847006b3cd545fd695e8b09af339134f
Reviewed-on: https://go-review.googlesource.com/22310Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/container/heap/heap_test.go

@@ -173,7 +173,7 @@ func TestRemove2(t *testing.T) {
 
 func BenchmarkDup(b *testing.B) {
 	const n = 10000
-	h := make(myHeap, n)
+	h := make(myHeap, 0, n)
 	for i := 0; i < b.N; i++ {
 		for j := 0; j < n; j++ {
 			Push(&h, 0) // all elements are the same