Fix flaky multi_alter_table_statements (#7321)

Sometimes multi_alter_table_statements would fail in CI like this:

```diff
 -- Verify that DROP NOT NULL works
 ALTER TABLE lineitem_alter ALTER COLUMN int_column2 DROP NOT NULL;
 SELECT "Column", "Type", "Modifiers" FROM table_desc WHERE relid='lineitem_alter'::regclass;
-     Column      |         Type          | Modifiers
----------------------------------------------------------------------
- l_orderkey      | bigint                | not null
- l_partkey       | integer               | not null
- l_suppkey       | integer               | not null
- l_linenumber    | integer               | not null
- l_quantity      | numeric(15,2)         | not null
- l_extendedprice | numeric(15,2)         | not null
- l_discount      | numeric(15,2)         | not null
- l_tax           | numeric(15,2)         | not null
- l_returnflag    | character(1)          | not null
- l_linestatus    | character(1)          | not null
- l_shipdate      | date                  | not null
- l_commitdate    | date                  | not null
- l_receiptdate   | date                  | not null
- l_shipinstruct  | character(25)         | not null
- l_shipmode      | character(10)         | not null
- l_comment       | character varying(44) | not null
- float_column    | double precision      | default 1
- date_column     | date                  |
- int_column1     | integer               |
- int_column2     | integer               |
- null_column     | integer               |
-(21 rows)
-
+ERROR:  schema "alter_table_add_column" does not exist
 -- COPY should succeed now
 SELECT master_create_empty_shard('lineitem_alter') as shardid \gset
 ```

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_alter_table_statements in parallel
with alter_table_add_column anymore.

This is another instance of the same issue as in #7294

src/test/regress/multi_1_schedule

@@ -162,7 +162,8 @@ test: with_executors with_join with_partitioning with_transactions with_dml
 # Tests around DDL statements run on distributed tables
 # ----------
 test: multi_index_statements
-test: multi_alter_table_statements alter_table_add_column
+test: multi_alter_table_statements
+test: alter_table_add_column
 test: multi_alter_table_add_constraints
 test: multi_alter_table_add_constraints_without_name
 test: multi_alter_table_add_foreign_key_without_name