add exception for simpletest to delete

Testing from the Simpletest module has the ability to create temporary users during automated browser tests to simulate logging in with specific permissions. Simpletest creates a user and usernode creates a matching usernode. When Simpletest is done with its test, it cleans up and tries to delete the user, but usernode disallows the deletion of the corresponding usernode. This patch creates an exception for the deletion of usernodes created by Simpletest, which are always prefixed with simpletest_ in the node title and the node owner's name.