macho: fix tests Fix a few bugs that are causing the tests to fail. Change-Id: Ie2d1b76ee4650a17092a0ff8aef004d118dabe84 Reviewed-on: https://go-review.googlesource.com/92419 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>