ipn/ipnlocal: update suggestExitNode to skip offline candidates and fix TestSetControlClientStatusAutoExitNode

TestSetControlClientStatusAutoExitNode is broken similarly to TestUpdateNetmapDeltaAutoExitNode
as suggestExitNode didn't previously check the online status of exit nodes, and similarly to the other test
it succeeded because the test itself is also broken.

However, it is easier to fix as it sends out a full netmap update rather than a delta peer update,
so it doesn't depend on the same refactoring as TestSetControlClientStatusAutoExitNode.

Updates #16455
Updates tailscale/corp#29969

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local.go

@@ -7433,7 +7433,7 @@ func suggestExitNode(report *netcheck.Report, netMap *netmap.NetworkMap, prevSug
 	}
 	candidates := make([]tailcfg.NodeView, 0, len(netMap.Peers))
 	for _, peer := range netMap.Peers {
-		if !peer.Valid() {
+		if !peer.Valid() || !peer.Online().Get() {
 			continue
 		}
 		if allowList != nil && !allowList.Contains(peer.StableID()) {