I suggest to split the tests like this, and also fix the spelling on Partial

@test.it('Partial tests 2x2')
def _():
    test.assert_equals(list(dithering(2, 2)),
                       [(0,0), (1,1), (1,0), (0,1)])
@test.it('Partial tests 4x4')
def _():        
    g = dithering(4, 4)
    for i in range(10):
      next(g)
    test.assert_equals(next(g), (3,0))