cmd/containerboot, net/dns/resolver: remove unused funcs in tests

staticcheck was complaining about it on a PR
I sent: https://github.com/tailscale/tailscale/actions/runs/22408882872/job/64876543467?pr=18804

And: https://github.com/tailscale/tailscale/actions/runs/22408882872/job/64876543475?pr=18804

Updates #cleanup
Updates #18157

Change-Id: I6225481f3aab9e43ef1920aa1a12e86c5073a638
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f