[INTERNAL] replaced JID with User objects

Replaced all occurrences of JID object with User objects in the Jupiter
test classes and test cases.

Change-Id: Ib503bf4a3712cb08c7a10f6d5c9cdd3d8c900589
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1624
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>