Remove the obsolete comment for RETURNING and put the test in a parallel...

Remove the obsolete comment for RETURNING and put the test in a parallel running group, following pg upstream.

src/test/regress/parallel_schedule

@@ -149,14 +149,11 @@ test: select_views portals_p2 cluster dependency guc bitmapops tsearch tsdicts f
 # NB: temp.sql does a reconnect which transiently uses 2 connections,
 # so keep this parallel group to at most 19 tests
 # ----------
-test: plancache limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table alter_extension sequence polymorphism rowtypes with xml gp_foreign_data
+test: plancache limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table alter_extension sequence polymorphism rowtypes returning with xml gp_foreign_data
 
 # large objects are not supported by GPDB
 # test: largeobject
 
-# INSERT RETURNING is not supported by GPDB
-test: returning
-
 # run stats by itself because its delay may be insufficient under heavy load
 #
 # 'stats' test has been disabled in GPDB, because it tries to set stats-related