cmd/tailscale/cli: [set] handle selectively modifying routes/exit node

Noticed this while debugging something else, we would reset all routes if
either `--advertise-exit-node` or `--advertise-routes` were set. This handles
correctly updating them.

Also added tests.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/local.go

@@ -2930,19 +2930,10 @@ func peerRoutes(peers []wgcfg.Peer, cgnatThreshold int) (routes []netip.Prefix)
 		routes = append(routes, cgNATIPs...)
 	}
 
-	sort.Slice(routes, func(i, j int) bool {
-		return ipPrefixLess(routes[i], routes[j])
-	})
+	tsaddr.SortPrefixes(routes)
 	return routes
 }
 
-func ipPrefixLess(ri, rj netip.Prefix) bool {
-	if ri.Addr() == rj.Addr() {
-		return ri.Bits() < rj.Bits()
-	}
-	return ri.Addr().Less(rj.Addr())
-}
-
 // routerConfig produces a router.Config from a wireguard config and IPN prefs.
 func (b *LocalBackend) routerConfig(cfg *wgcfg.Config, prefs ipn.PrefsView, oneCGNATRoute bool) *router.Config {
 	singleRouteThreshold := 10_000