wgengine/{,magicsock},tstest/natlab/vmtest: send disco on cached netmap (#19878)

Originally found when adding tests for working with cached netmaps, and
finding the added tests to be flakey.

When working off of a cached netmap, if a node exists in the cached
netmap but does not yet have any endpoints, DERP connections are
available but not direct ones. By sending callMeMaybe to nodes
without endpoints in the cached netmap, we can establish direct
connections for this edge case.

Aditionally, ensure that TSMP disco advert messages are not sent if the
endpoint does not have a valid address yet.

Fixes #19843
Updates #19597

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/userspace.go

@@ -614,6 +614,9 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 	})
 	var tsmpRequestGroup singleflight.Group[netip.Addr, struct{}]
 	eventbus.SubscribeFunc(ec, func(req magicsock.NewDiscoKeyAvailable) {
+		if !req.NodeFirstAddr.IsValid() {
+			return
+		}
 		go tsmpRequestGroup.Do(req.NodeFirstAddr, func() (struct{}, error) {
 			e.sendTSMPDiscoAdvertisement(req.NodeFirstAddr)
 			e.logf("wgengine: sending TSMP disco key advertisement to %v", req.NodeFirstAddr)