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

[tbpg]

What version of Go are you using (go version)?

go version go1.9 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"

What did you do?

I read https://golang.org/src/container/heap/heap_test.go.

What did you expect to see?

I expected verify() to use t.Helper(), making test failures print the line of the call to verify().

What did you see instead?

Failures print the line where t.Errorf() was called.

[gopherbot]

Change https://golang.org/cl/63570 mentions this issue: container/heap: call t.Helper() in verify()