Cleanup/expand the filter/query tests

This patch cleans up duplicate code in Test.Ganeti.Query.Filter and
then adds a test for names consistency with Python's code behaviour
(stable ordering for simple filters and otherwise niceSort'ed
ordering).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>