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>

ipn/ipnlocal/profiles.go

@@ -21,6 +21,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/eventbus"
 )
 
 var debug = envknob.RegisterBool("TS_DEBUG_PROFILES")
@@ -838,7 +839,9 @@ func (pm *profileManager) CurrentPrefs() ipn.PrefsView {
 
 // ReadStartupPrefsForTest reads the startup prefs from disk. It is only used for testing.
 func ReadStartupPrefsForTest(logf logger.Logf, store ipn.StateStore) (ipn.PrefsView, error) {
-	ht := new(health.Tracker) // in tests, don't care about the health status
+	bus := eventbus.New()
+	defer bus.Close()
+	ht := health.NewTracker(bus) // in tests, don't care about the health status
 	pm, err := newProfileManager(store, logf, ht)
 	if err != nil {
 		return ipn.PrefsView{}, err