Include fixtures to prevent foreign key violation.

When some other tests use `:authors` fixtures, the authors table in db has the following content:

```bash
+----+-------+-------------------+-------------------------+-----------------+-------------------+
| id | name  | author_address_id | author_address_extra_id | organization_id | owned_essay_id    |
+----+-------+-------------------+-------------------------+-----------------+-------------------+
|  1 | David |                 1 |                       2 | No Such Agency  | A Modest Proposal |
|  2 | Mary  |              NULL |                    NULL | NULL            | NULL              |
|  3 | Bob   |              NULL |                    NULL | NULL            | NULL              |
+----+-------+-------------------+-------------------------+-----------------+-------------------+
```

Destroying address in this test case would therefore result in foreign key violation error. With these two fixtures loaded before the test, we can prevent the error.

cc @senny