Adjust parallel_schedule with event triggers on authenticated login

Event triggers on authenticated login could catch the connection of the
concurrent test.  In order to prevent this side effect we need to run
the event_trigger test alone.

Discussion: https://postgr.es/m/flat/CAMEv5_sS5G7K1PCV2oBx5+1NF1ZneJ6C5Z_xV_SWsZMukdFZiA@mail.gmail.com
Author: Mikhail A. Gribkov
Reviewed-by: Aleksander Alekseev

src/test/regress/parallel_schedule

@@ -121,10 +121,12 @@ test: plancache limit plpgsql copy2 temp domain rangefuncs prepare conversion tr
 # ----------
 test: partition_join partition_prune reloptions hash_part indexing partition_aggregate partition_info tuplesort explain compression memoize stats
 
-# event_trigger depends on create_am and cannot run concurrently with
-# any test that runs DDL
-# oidjoins is read-only, though, and should run late for best coverage
-test: event_trigger oidjoins
+# event_trigger cannot run concurrently with any other tests because
+# on-login event handling could catch connection of a concurrent test.
+test: event_trigger
+
+# oidjoins should run late for best coverage
+test: oidjoins
 
 # this test also uses event triggers, so likewise run it by itself
 test: fast_default