servertest: cover via-grant exit-node visibility end-to-end

TestGrantViaExitNodeInternetVisibility boots a server, applies a
policy that scopes autogroup:internet to a tag, registers a tagged
exit advertiser and a regular client, and asserts the client's netmap
surfaces the exit node with 0.0.0.0/0 and ::/0 in AllowedIPs — the
substrate the Tailscale client reads to populate
`tailscale exit-node list`.

TestGrantViaExitNodeNoFilterRules retains its assertion (literal /0
absent from the exit node's PacketFilter, matching SaaS PacketFilter
encoding); only its docstring is updated to reflect that the exit
node now does receive a TheInternet-shaped rule, just not the
literal /0 form.

Updates #3233