Ignore disallow_database_ddl_feature_flags feature flag during partitioned table creation in specs

Summary

We have spec support method MigrationHelpers#partitioned_table which creates a partitioned table for specific batched background migration spec, however it fails to sync dynamic partitions because of :disallow_database_ddl_feature_flags feature flag check which is enabled in specs. As a result no partitions is created and table is not usable.

Proposal

Override disallow_database_ddl_feature_flags feature flag in MigrationHelpers#partitioned_table