tests: fix out of bounds access in the rx-event test

tests: fix out of bounds access in the rx-event test

Use the NUMEVENTS symbol which defines the array size instead of an
incorrect hard coded number when checking if a second event can be added
to be fired at the same time.  This fixes a potential out of bounds
access of the event test array.

Also update the comment which incorrectly mentions the incorrect number
of events in the test.

Change-Id: I4f993b42e53e7e6a42fa31302fd1baa70e9f5041
Reviewed-on: https://gerrit.openafs.org/12762
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>