net/netcheck: deflake (maybe) magicsock's TestNewConn

Updates #6207

Change-Id: I51d200d0b42b9a1ef799d0abfc8d4bd871c50cf2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f