Update MY_Model::delete tests for 668247876b0f2d4fce10625eb703758e8d422eb2

Changes to the delete methods in
668247876b0f2d4fce10625eb703758e8d422eb2 resulted in failures in the
tests. Since the tests did not break based on the function results, I've
updated the tests to reflect the changes in the model.

One item I did not fix is that the tests don't handle method chaining on
$this->db properly, so the tests error out when they encounter any use
of this feature.