avoid passing objects we want to go away to Test::More::is

Test::Simple 1.302178 to 1.302182 have a bug where when running under
Devel::Cover, objects passed to Test::More::is will be captured and held
until process end. When testing weakening, we expect an object to go
away. Avoid passing the object to 'is' to prevent it from ever being
captured.

t/cmop/instance.t

@@ -97,12 +97,12 @@ $mi_foo->set_slot_value( $i_foo, "moosen", $ref );
 $mi_foo->weaken_slot_value( $i_foo, "moosen" );
 
 ok( isweak($i_foo->{moosen}), '... white box test of weaken' );
-is( $mi_foo->get_slot_value( $i_foo, "moosen" ), $ref, "weak value is fetchable" );
+ok( ($mi_foo->get_slot_value( $i_foo, "moosen" ) || 0) == $ref, "weak value is fetchable" );
 ok( !isweak($mi_foo->get_slot_value( $i_foo, "moosen" )), "return value not weak" );
 
 undef $ref;
 
-is( $mi_foo->get_slot_value( $i_foo, "moosen" ), undef, "weak value destroyed" );
+ok( !defined $mi_foo->get_slot_value( $i_foo, "moosen" ), "weak value destroyed" );
 
 $ref = [];