Fix a race condition that happened in the pop3 unit tests:

1. The pop3 resource would use ItemCreateJobs to add items to a collection (with maildir as backend)
2. The unit test would immediatley fetch those items from Akonadi. Those items have no remote id yet,
   because the maildir resource has not yet processed them.
3. The maildir resource would in the meantime add a remote id to the items
4. The unit test would delete the items, but with the item references from step 2, which don't have a
   remote id yet. the delete job would start a modify job, which would set the remote id of the items
   to an empty string.
5. The items can't be deleted from the maildir anymore, as they have no remote id anymore.

Fix this by setting the remote id to a null string in itemfetchjob (step 2), so that subsequent item
modify jobs will not try to change the remote id anymore.

Add very simplistic test for this.

svn path=/trunk/KDE/kdepimlibs/; revision=952795