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

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/63570
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>