[BUG #8500] Fixes test with impossible success in qx.ui.table.model.Filtered class

Hi,
I opened a bug report here with the details of the bug : http://bugzilla.qooxdoo.org/show_bug.cgi?id=8500
I offer a fix patch + test case (I'm not sure of the placement of the test file, It's my first pull request).
Thanks