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/extension_host_test.go

@@ -32,6 +32,7 @@ import (
 	"tailscale.com/types/lazy"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/persist"
+	"tailscale.com/util/eventbus/eventbustest"
 	"tailscale.com/util/must"
 )
 
@@ -847,7 +848,7 @@ func TestBackgroundProfileResolver(t *testing.T) {
 
 			// Create a new profile manager and add the profiles to it.
 			// We expose the profile manager to the extensions via the read-only [ipnext.ProfileStore] interface.
-			pm := must.Get(newProfileManager(new(mem.Store), t.Logf, new(health.Tracker)))
+			pm := must.Get(newProfileManager(new(mem.Store), t.Logf, health.NewTracker(eventbustest.NewBus(t))))
 			for i, p := range tt.profiles {
 				// Generate a unique ID and key for each profile,
 				// unless the profile already has them set