Enlarging assignment test script field

Take care at deployment time because I couldn't test the migration process.  I gathered information from (https://github.com/datamapper/dm-migrations/blob/8bfcec08286a12ceee1bc3e5a01da3b5b7d4a74d/lib/dm-migrations/sql/table_modifier.rb#L40, https://github.com/datamapper/dm-migrations) in order to validate the migration action.