golinter fix (#1807)
golinter fix (#1807)

This fix fixes golinter warning:
```
plugin/tls/tls_test.go:1::warning: file is not goimported (goimports)
```

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