Run stats test separately, not as part of a parallel group, to avoid

possible failures due to stats collector not updating fast enough under
heavy load.  Per report from Jeremy Yoder.

src/test/regress/parallel_schedule

@@ -74,4 +74,7 @@ test: select_views portals_p2 rules foreign_key cluster
 # The sixth group of parallel test
 # ----------
 # "plpgsql" cannot run concurrently with "rules"
-test: limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table sequence polymorphism stats
+test: limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table sequence polymorphism
+
+# run stats by itself because its delay may be insufficient under heavy load
+test: stats