Validator should check `setSpec` values in response to ListSets with a set specification

[zimeon]

Currently the validator doesn't look at the setSpec values in the ListSets response as part of test_list_sets. Errors can be reported, somewhat confusingly, in the later GetRecord test.

[zimeon]

See thread: https://groups.google.com/forum/#!topic/oai-pmh/hDymZcLgfq0