Create empty file to fix go build ./...

Go 1.3 broke go build ./... for packages containing test-only
directories. This adds a non-test Go file with no content (besides a
comment explaining its purpose) to the nsqd/test folder, allowing the
repository to once again be built with go build ./...

See issue #409 for more information and issue #381 for links.
might this interact poorly with `go-nsq`?

Let's name this something that doesn't conflict with an *actual* package name...

Perhaps `nsqdonteverimportme` or something equally obvious?