Extend the Expect method of a Watcher to allow filter functions that report
only an error value, and which "pass" when the reported error is nil.
Updates #15160
Change-Id: I582d804554bd1066a9e499c1f3992d068c9e8148
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>
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>