util/eventbus: add test helpers to simplify testing events (#16294)

Instead of every module having to come up with a set of test methods for
the event bus, this handful of test helpers hides a lot of the needed
setup for the testing of the event bus.

The tests in portmapper is also ported over to the new helpers.

Updates #15160

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

net/portmapper/portmapper.go

@@ -515,7 +515,7 @@ func (c *Client) createMapping() {
 			GoodUntil: mapping.GoodUntil(),
 		})
 	}
-	if c.onChange != nil {
+	if c.onChange != nil && c.pubClient == nil {
 		go c.onChange()
 	}
 }