golint fix (#2504)
golint fix (#2504)

This PR is a small golint fix:
```
coredns/plugin/test/helpers.go
Line 283: warning: redundant if ...; err != nil check, just return error instead. (golint)
```

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
johnbelamaric/coredns,miekg/coredns,ii/coredns,coredns/coredns,chrisohaver/coredns,yongtang/coredns,miekg/coredns,johnbelamaric/coredns,ii/coredns