tstest/natlab/vnet: standardize on 1-based naming of nodes, networks, MACs

We had a mix of 0-based and 1-based nodes and MACs in logs.

Updates #13038

Change-Id: I36d1b00f7f94b37b4ae2cd439bcdc5dbee6eda4d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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