Before synctest, timers was needed to allow the events to flow into the
test bus. There is still a timer, but this one is not derived from the
test deadline and it is mostly arbitrary as synctest will render it
practically non-existent.
With this approach, tests that do not need to test for the absence of
events do not rely on synctest.
Updates #15160
Signed-off-by: Claus Lensbøl <claus@tailscale.com>