Suppress DETAIL output from an event_trigger test.
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.