ipn/ipnlocal: add a test for recent WhoIs regression

This would've prevented #9470.

This used to pass, fails as of 9538e9f970, and passes again
once #9472 is in.

Updates #9470

Change-Id: Iab97666f7a318432fb3b6372a177ab50c55d4697
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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