tunnel: Fix resolver file permissions, add DNS forwarding test

[tstromberg]

A couple of changes herein to make a complete fix for #7747

route_darwin.go:

• Make resolver file world readable
• better error messages and logging
• writeResolverFile failures now emit an error rather than disabling the tunnel

fn_tunnel_cmd.go:

• Make it work on Darwin by using a sudo test command that returns 0.
• Add testing for DNS forwarding
• Make output easier to understand

[k8s-ci-robot]

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[tstromberg]

/ok-to-test

[minikube-pr-bot]

kvm2 Driver
Times for minikube: [66.09637953100001 63.954625075 64.419402144]
Average time for minikube: 64.82346891666667

Times for Minikube (PR 7753): [62.91173922300001 63.43298633 64.33800803]
Average time for Minikube (PR 7753): 63.560911194333336

Averages Time Per Log

+--------------------------------+-----------+--------------------+
|              LOG               | MINIKUBE  | MINIKUBE (PR 7753) |
+--------------------------------+-----------+--------------------+
| * minikube v1.9.2 on Debian    |  0.061700 |           0.057970 |
|                           9.11 |           |                    |
| * Using the kvm2 driver based  |  0.019465 |           0.018736 |
| on existing profile            |           |                    |
| * Starting control plane node  |  0.003040 |           0.003011 |
| minikube in cluster minikube   |           |                    |
| * Creating kvm2 VM (CPUs=2,    | 40.737042 |          40.953401 |
| Memory=3700MB, Disk=20000MB)   |           |                    |
| ...                            |           |                    |
| * Preparing Kubernetes v1.18.0 | 21.953379 |          20.542498 |
| on Docker 19.03.8 ...          |           |                    |
| * Enabling addons:             |  1.950467 |           1.920174 |
| default-storageclass,          |           |                    |
| storage-provisioner            |           |                    |
| * Done! kubectl is now         |  0.094108 |           0.061545 |
| configured to use "minikube"   |           |                    |
|                                |  0.004268 |           0.003575 |
+--------------------------------+-----------+--------------------+

docker Driver
Times for minikube: [25.474837827 25.464081514999997 24.843463342000003]
Average time for minikube: 25.260794227999998

Times for Minikube (PR 7753): [26.961308721000005 26.123503487000004 24.297481208999997]
Average time for Minikube (PR 7753): 25.794097805666667

Averages Time Per Log

+----------------------------------------+-----------+--------------------+
|                  LOG                   | MINIKUBE  | MINIKUBE (PR 7753) |
+----------------------------------------+-----------+--------------------+
| * minikube v1.9.2 on Debian            |  0.069469 |           0.070516 |
|                                   9.11 |           |                    |
| * Using the docker driver              |  0.002548 |           0.002173 |
| based on existing profile              |           |                    |
| * Starting control plane node          |  0.054302 |           0.054947 |
| minikube in cluster minikube           |           |                    |
| * Creating docker container            |  7.235732 |           7.205555 |
| (CPUs=2, Memory=3700MB) ...            |           |                    |
| * Preparing Kubernetes v1.18.0         |  0.000144 |           0.000249 |
| on Docker 19.03.2 ...                  |           |                    |
|   -                                    | 16.943661 |          17.269219 |
| kubeadm.pod-network-cidr=10.244.0.0/16 |           |                    |
| * Enabling addons:                     |  0.892604 |           1.125567 |
| default-storageclass,                  |           |                    |
| storage-provisioner                    |           |                    |
| * Done! kubectl is now                 |  0.058059 |           0.061285 |
| configured to use "minikube"           |           |                    |
|                                        |  0.004275 |           0.004587 |
+----------------------------------------+-----------+--------------------+