Fix 6428 - authenticate() always fails on IBMi when using DB table-based authentication

Here is the issue report which this pull request addresses - https://github.com/zendframework/zf2/issues/6428

One-line addition to the AbstractAdapter, a set of unit tests specific for IBM i / DB2 (run on the IBM, of course, against a DB2 table), and some additional TestConfiguration constants defined for controlling the tests.

Bear with me: this is my first ZF2 pull request!