Fix flaky multi_prepare_plsql (#7346)

Simple need of an `ORDER BY` clause

Ran into this twice this week already!

https://github.com/citusdata/citus/actions/runs/6849701315/attempts/1#summary-18622563506

https://github.com/citusdata/citus/actions/runs/6875051160/attempts/1#summary-18698009952

```diff
 SELECT nspname, typname FROM pg_type JOIN pg_namespace ON pg_namespace.oid = pg_type.typnamespace WHERE typname = 'prepare_ddl_type_backup';
    nspname   |         typname         
 -------------+-------------------------
- public      | prepare_ddl_type_backup
  otherschema | prepare_ddl_type_backup
+ public      | prepare_ddl_type_backup
 (2 rows)
```

src/test/regress/citus_tests/run_test.py

@@ -198,6 +198,7 @@ DEPS = {
         ["multi_create_table", "multi_create_users", "multi_multiuser_load_data"],
         repeatable=False,
     ),
+    "multi_prepare_plsql": TestDeps("base_schedule"),
 }
 
 

src/test/regress/expected/multi_prepare_plsql.out

@@ -1317,11 +1317,11 @@ SELECT type_ddl_plpgsql();
 (1 row)
 
 -- find all renamed types to verify the schema name didn't leak, nor a crash happened
-SELECT nspname, typname FROM pg_type JOIN pg_namespace ON pg_namespace.oid = pg_type.typnamespace WHERE typname = 'prepare_ddl_type_backup';
+SELECT nspname, typname FROM pg_type JOIN pg_namespace ON pg_namespace.oid = pg_type.typnamespace WHERE typname = 'prepare_ddl_type_backup' ORDER BY 1;
    nspname   |         typname
 ---------------------------------------------------------------------
- public      | prepare_ddl_type_backup
  otherschema | prepare_ddl_type_backup
+ public      | prepare_ddl_type_backup
 (2 rows)
 
 DROP TYPE prepare_ddl_type_backup;
@@ -1332,6 +1332,7 @@ DROP FUNCTION ddl_in_plpgsql();
 DROP FUNCTION copy_in_plpgsql();
 DROP TABLE prepare_ddl;
 DROP TABLE local_ddl;
+DROP TABLE plpgsql_table;
 DROP SCHEMA otherschema;
 -- clean-up functions
 DROP FUNCTION plpgsql_test_1();

src/test/regress/multi_schedule

@@ -83,7 +83,8 @@ test: forcedelegation_functions
 # this should be run alone as it gets too many clients
 test: join_pushdown
 test: multi_subquery_union multi_subquery_in_where_clause multi_subquery_misc statement_cancel_error_message
-test: multi_agg_distinct multi_limit_clause_approximate multi_outer_join_reference multi_single_relation_subquery multi_prepare_plsql set_role_in_transaction
+test: multi_agg_distinct
+test: multi_limit_clause_approximate multi_outer_join_reference multi_single_relation_subquery multi_prepare_plsql set_role_in_transaction
 test: multi_reference_table multi_select_for_update relation_access_tracking pg13_with_ties
 test: custom_aggregate_support aggregate_support tdigest_aggregate_support
 test: multi_average_expression multi_working_columns multi_having_pushdown having_subquery

src/test/regress/sql/multi_prepare_plsql.sql

@@ -624,7 +624,7 @@ CREATE TYPE prepare_ddl_type AS (x int, y int);
 SELECT type_ddl_plpgsql();
 
 -- find all renamed types to verify the schema name didn't leak, nor a crash happened
-SELECT nspname, typname FROM pg_type JOIN pg_namespace ON pg_namespace.oid = pg_type.typnamespace WHERE typname = 'prepare_ddl_type_backup';
+SELECT nspname, typname FROM pg_type JOIN pg_namespace ON pg_namespace.oid = pg_type.typnamespace WHERE typname = 'prepare_ddl_type_backup' ORDER BY 1;
 
 DROP TYPE prepare_ddl_type_backup;
 RESET search_path;
@@ -635,6 +635,7 @@ DROP FUNCTION ddl_in_plpgsql();
 DROP FUNCTION copy_in_plpgsql();
 DROP TABLE prepare_ddl;
 DROP TABLE local_ddl;
+DROP TABLE plpgsql_table;
 DROP SCHEMA otherschema;
 
 -- clean-up functions