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>

util/eventbus/eventbustest/eventbustest.go

@@ -100,7 +100,7 @@ func Expect(tw *Watcher, filters ...any) error {
 		case <-time.After(tw.TimeOut):
 			return fmt.Errorf(
 				"timed out waiting for event, saw %d events, %d was expected",
-				eventCount, head)
+				eventCount, len(filters))
 		case <-tw.chDone:
 			return errors.New("watcher closed while waiting for events")
 		}
@@ -138,7 +138,7 @@ func ExpectExactly(tw *Watcher, filters ...any) error {
 		case <-time.After(tw.TimeOut):
 			return fmt.Errorf(
 				"timed out waiting for event, saw %d events, %d was expected",
-				eventCount, pos)
+				eventCount, len(filters))
 		case <-tw.chDone:
 			return errors.New("watcher closed while waiting for events")
 		}