tstest/natlab/vnet: rename some things for clarity

The bad naming (which had only been half updated with the IPv6
changes) tripped me up in the earlier change.

Updates #13038

Change-Id: I65ce07c167e8219d35b87e1f4bf61aab4cac31ff
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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