Adjust rx-event test to exercise cancel/fire race

Adjust rx-event test to exercise cancel/fire race

We currently do not properly handle the case where a thread runs
rxevent_Cancel() in parallel with the event-handler thread attempting
to fire that event, but the test suite only picked up on this issue
in a handful of the Debian automated builds (somewhat less-resourced
ones, perhaps).

Modify the event scheduling algorithm in the test so as to create a
larger chunk of events scheduled to fire "right away" and thereby
exercise the race condition more often when we proceed to cancel
a quarter of events "right away".

Reviewed-on: https://gerrit.openafs.org/12755
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit bdb509fb1d8e0fdca05dffecdbcbf60a95ea502e)

Change-Id: I27cebed3c2c3daff10b8d3f5f6f949e667791a72
Reviewed-on: https://gerrit.openafs.org/12774
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>