health,ipn/ipnlocal: introduce eventbus in heath.Tracker (#17085)

The Tracker was using direct callbacks to ipnlocal. This PR moves those
to be triggered via the eventbus.

Additionally, the eventbus is now closed on exit from tailscaled
explicitly, and health is now a SubSystem in tsd.

Updates #15160

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

net/dns/resolver/tsdns_test.go

@@ -31,7 +31,7 @@ import (
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
-	"tailscale.com/util/eventbus"
+	"tailscale.com/util/eventbus/eventbustest"
 )
 
 var (
@@ -356,7 +356,7 @@ func newResolver(t testing.TB) *Resolver {
 	return New(t.Logf,
 		nil, // no link selector
 		tsdial.NewDialer(netmon.NewStatic()),
-		new(health.Tracker),
+		health.NewTracker(eventbustest.NewBus(t)),
 		nil, // no control knobs
 	)
 }
@@ -1060,8 +1060,7 @@ func TestForwardLinkSelection(t *testing.T) {
 	// routes differently.
 	specialIP := netaddr.IPv4(1, 2, 3, 4)
 
-	bus := eventbus.New()
-	defer bus.Close()
+	bus := eventbustest.NewBus(t)
 
 	netMon, err := netmon.New(bus, logger.WithPrefix(t.Logf, ".... netmon: "))
 	if err != nil {
@@ -1074,7 +1073,7 @@ func TestForwardLinkSelection(t *testing.T) {
 			return "special"
 		}
 		return ""
-	}), new(tsdial.Dialer), new(health.Tracker), nil /* no control knobs */)
+	}), new(tsdial.Dialer), health.NewTracker(bus), nil /* no control knobs */)
 
 	// Test non-special IP.
 	if got, err := fwd.packetListener(netip.Addr{}); err != nil {