Add allow_segment_DML to utility test function

Author: reshke

src/test/regress/sql/matview.sql

@@ -241,7 +241,7 @@ DROP ROLE regress_user_mvtest;
 CREATE OR REPLACE FUNCTION mvtest_drop_the_index()
   RETURNS bool AS $$
 BEGIN
-  EXECUTE 'DROP INDEX IF EXISTS mvtest_drop_idx';
+  EXECUTE 'SET allow_segment_DML = ON;DROP INDEX IF EXISTS mvtest_drop_idx';
   RETURN true;
 END;
 $$ LANGUAGE plpgsql;