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/forwarder_test.go

@@ -29,7 +29,7 @@ import (
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tstest"
 	"tailscale.com/types/dnstype"
-	"tailscale.com/util/eventbus"
+	"tailscale.com/util/eventbus/eventbustest"
 )
 
 func (rr resolverAndDelay) String() string {
@@ -455,8 +455,7 @@ func makeLargeResponse(tb testing.TB, domain string) (request, response []byte)
 
 func runTestQuery(tb testing.TB, request []byte, modify func(*forwarder), ports ...uint16) ([]byte, error) {
 	logf := tstest.WhileTestRunningLogger(tb)
-	bus := eventbus.New()
-	defer bus.Close()
+	bus := eventbustest.NewBus(tb)
 	netMon, err := netmon.New(bus, logf)
 	if err != nil {
 		tb.Fatal(err)
@@ -465,7 +464,7 @@ func runTestQuery(tb testing.TB, request []byte, modify func(*forwarder), ports
 	var dialer tsdial.Dialer
 	dialer.SetNetMon(netMon)
 
-	fwd := newForwarder(logf, netMon, nil, &dialer, new(health.Tracker), nil)
+	fwd := newForwarder(logf, netMon, nil, &dialer, health.NewTracker(bus), nil)
 	if modify != nil {
 		modify(fwd)
 	}