Improved robustness of object destruction.

Squashed commit of the following:

commit 55d412ecf6738d654e9c3e38c33844e550983484
Author: Moritz Lenz <mlenz@noris.net>
Date:   Tue Mar 4 14:07:23 2014 +0100

    Properly clean out object manager at the end of the test

    otherwise the UnitTest's $Kernel::OM will take over at some point

commit 19c63a1ffcfe73defb0aa5ce4e1bbff8620f08ab
Author: Moritz Lenz <mlenz@noris.net>
Date:   Tue Mar 4 13:58:40 2014 +0100

    forgot to add Dummy2.pm

commit 789f8c6c8872531ce44053183c82a8917a164906
Author: Moritz Lenz <mlenz@noris.net>
Date:   Tue Mar 4 13:52:38 2014 +0100

    Improved robustness of object destruction

    Some objects create new objects in their destructor; thus
    several passes might be necessary to get rid of all referenced
    objects.