Also test the dark collection

t/01.utils/collection.t

@@ -52,9 +52,11 @@ is(@$ref, $i, 'Same size - there can be only 1 fox');
 $collection->add_once('human');
 is(@$ref, ++$i, 'Addition - Someone comes in');
 
-$collection->forget('fox');
+$collection->forget_and_mark_for_deletion('fox');
 is(@$ref, --$i, 'one less element - the fox is gone: we\'re safe !');
 
+ok($collection->dark_collection->present('fox'));
+
 $collection = Bio::EnsEMBL::Hive::Utils::Collection->new([{ foo => undef}]);
 $collection->add({ foo => undef });
 $collection->add({ bar => 'foobar' });