net/portmapper: fix test flakes from logging after test done

Fixes #15794

Change-Id: Ic22aa99acb10fdb6dc5f0b6482e722e48237703c
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