tstest/natlab/vnet: fix one-by-one from earlier numbering change

84adfa1ba3 made MAC addresses 1-based too, but didn't adjust this IP address
calculation which was based on the MAC address

Updates #13038

Change-Id: Idc112b303b0b85f41fe51fd61ce1c0d8a3f0f57e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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