p2p/simulations: encapsulate Node.Up field so we avoid data races

The Node.Up field was accessed concurrently without "proper" locking.
There was a lock on Network and that was used sometimes to access
the  field. Other times the locking was missed and we had
a data race.

For example: https://github.com/ethereum/go-ethereum/pull/18464
The case above was solved, but there were still intermittent/hard to
reproduce races. So let's solve the issue permanently.

resolves: ethersphere/go-ethereum#1146

swarm/network/simulations/overlay_test.go

@@ -178,7 +178,7 @@ func watchSimEvents(net *simulations.Network, ctx context.Context, trigger chan
 		case ev := <-events:
 			//only catch node up events
 			if ev.Type == simulations.EventTypeNode {
-				if ev.Node.Up {
+				if ev.Node.Up() {
 					log.Debug("got node up event", "event", ev, "node", ev.Node.Config.ID)
 					select {
 					case trigger <- ev.Node.Config.ID: