Add allow_segment_DML to utility test function

Author: reshke

src/test/regress/sql/matview.sql

@@ -238,13 +238,15 @@ DROP ROLE regress_user_mvtest;
 
 -- Concurrent refresh requires a unique index on the materialized
 -- view. Test what happens if it's dropped during the refresh.
+SET allow_segment_DML = ON;
 CREATE OR REPLACE FUNCTION mvtest_drop_the_index()
   RETURNS bool AS $$
 BEGIN
   EXECUTE 'DROP INDEX IF EXISTS mvtest_drop_idx';
   RETURN true;
 END;
 $$ LANGUAGE plpgsql;
+RESET allow_segment_DML;
 
 CREATE MATERIALIZED VIEW drop_idx_matview AS
   SELECT 1 as i WHERE mvtest_drop_the_index();