container/heap: call t.Helper() in verify()

I modified verify() to fail every time to test the change. Before adding
t.Helper() (line 37 is in verify()):
/.../go/src/container/heap/heap_test.go:37: forced failure
FAIL

Afer adding t.Helper() (line 67 is where verify() is called):
/.../go/src/container/heap/heap_test.go:67: forced failure
FAIL

Fixes #21863

Change-Id: I46f0c8ec413cc664358c568fc53e48bb4a1d03d0
Reviewed-on: https://go-review.googlesource.com/63570Reviewed-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

@@ -33,6 +33,7 @@ func (h *myHeap) Push(v interface{}) {
 }
 
 func (h myHeap) verify(t *testing.T, i int) {
+	t.Helper()
 	n := h.Len()
 	j1 := 2*i + 1
 	j2 := 2*i + 2