net: always set the servers field to defaultNS

net: always set the servers field to defaultNS

When the field is parsed from a file, it would never
have len(servers) == 0, lets enforce that and update
test cases where we had it wrong.

Change-Id: I7fa6ebcf63b9fe20fbbf791113ca948d6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/743020
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>