7.x automatic deletion of test objects, assertDatastreams return values.

Two things in this update to the IslandoraWebTestCase:

* Most importantly, there's now a function to delete all objects created by a user. Plus, when you create a user for simpletest's internal browser using $this->createUser(), that user is automatically added to a list of test users whose objects will automatically be purged at the end of the test.
* Secondly, assertDatastreams() now returns TRUE if all datastreams were present, or FALSE if one or more were missing.