Fix flaky multi_reference_table test (#7294)

Sometimes multi_reference_table failed in CI like this:

```diff
 \c - - - :master_port
 DROP INDEX reference_schema.reference_index_2;
 \c - - - :worker_1_port
 SELECT "Column", "Type", "Modifiers" FROM table_desc WHERE relid='reference_schema.reference_table_ddl_1250019'::regclass;
- Column  |            Type             |  Modifiers
----------------------------------------------------------------------
- value_2 | double precision            | default 25.0
- value_3 | text                        | not null
- value_4 | timestamp without time zone |
- value_5 | double precision            |
-(4 rows)
-
+ERROR:  schema "citus_local_table_queries" does not exist
 \di reference_schema.reference_index_2*
           List of relations
  Schema | Name | Type | Owner | Table
```

Source:
https://github.com/citusdata/citus/actions/runs/6707535961/attempts/2#summary-18226879513

Reading from table_desc apparantly has an issue that if the schema gets
deleted from one of the items, while it is being read that we get such
an error.

This change fixes that by not running multi_reference_table in parallel
with citus_local_tables_queries anymore.

src/test/regress/multi_1_schedule

@@ -298,7 +298,8 @@ test: replicate_reference_tables_to_coordinator
 test: citus_local_tables
 test: mixed_relkind_tests
 test: multi_row_router_insert create_distributed_table_concurrently
-test: multi_reference_table citus_local_tables_queries
+test: multi_reference_table
+test: citus_local_tables_queries
 test: citus_local_table_triggers
 test: coordinator_shouldhaveshards
 test: local_shard_utility_command_execution