Suppress 3 lines of unstable DETAIL output from a DROP ROLE statement in
event_trigger.sql.  This is further cleanup for commit dd299df8.

Note that the event_trigger test instability issue is very similar to
the recently suppressed foreign_data test instability issue.  Both
issues involve DETAIL output for a DROP ROLE statement that needed to be
changed as part of dd299df8.

Per buildfarm member macaque.