tstest/integration/testcontrol: de-flake TestUserMetricsRouteGauges

SetSubnetRoutes was not sending update notifications to nodes when their
approved routes changed, causing nodes to not fetch updated netmaps with
PrimaryRoutes populated. This resulted in TestUserMetricsRouteGauges
flaking because it waited for PrimaryRoutes to be set, which only happened
if the node happened to poll for other reasons.

Now send updateSelfChanged notification to affected nodes so they fetch
an updated netmap immediately.

Fixes #17962

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -464,6 +464,9 @@ func (s *Server) SetSubnetRoutes(nodeKey key.NodePublic, routes []netip.Prefix)
 	defer s.mu.Unlock()
 	s.logf("Setting subnet routes for %s: %v", nodeKey.ShortString(), routes)
 	mak.Set(&s.nodeSubnetRoutes, nodeKey, routes)
+	if node, ok := s.nodes[nodeKey]; ok {
+		sendUpdate(s.updates[node.ID], updateSelfChanged)
+	}
 }
 
 // MasqueradePair is a pair of nodes and the IP address that the