tests: Give more leeway in rx/event-t

tests: Give more leeway in rx/event-t

Currently, the rx/event-t tests schedule a bunch of events up to 3
seconds in the future, and then we sleep for 3 seconds to give them a
chance to run. Since we're cutting it so close, this can rarely result
in a few events not being run (observed occasionally on FreeBSD 12.1,
where we failed to run about 3 events out of 10000).

To avoid this, just sleep for 4 seconds instead of 3. Also print out a
little more info regarding the number of fired/cancelled events, so we
can see the event count when it's wrong.

Change-Id: I6269bea2c245aeed00c129ff638423d0fa81ad23
Reviewed-on: https://gerrit.openafs.org/14160
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>