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>

util/eventbus/doc.go

@@ -89,4 +89,14 @@
 // The [Debugger], obtained through [Bus.Debugger], provides
 // introspection facilities to monitor events flowing through the bus,
 // and inspect publisher and subscriber state.
+//
+// Additionally, a debug command exists for monitoring the eventbus:
+//
+//	tailscale debug daemon-bus-events
+//
+// # Testing facilities
+//
+// Helpers for testing code with the eventbus can be found in:
+//
+//	eventbus/eventbustest
 package eventbus