It should be noted, though, that it is not "the only comprehensive testing tool which can run tests on every level of the OS". The NixOS GNU/Linux distribution has had such a feature for a couple of years, as documented in its manual: http://hydra.nixos.org/job/nixos/trunk/manual/latest/down... .
In fact, NixOS has a set of whole-system tests that are continuously run as changes are made to the distribution, and whose results can be seen at http://hydra.nixos.org/jobset/nixos/trunk .
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