And convert all callers over to the methods that check SelfNode.
Now we don't have multiple ways to express things in tests (setting
fields on SelfNode vs NetworkMap, sometimes inconsistently) and don't
have multiple ways to check those two fields (often only checking one
or the other).
Updates #9443
Change-Id: I2d7ba1cf6556142d219fae2be6f484f528756e3c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>