test/pybind/rbd: don't ignore from_snapshot in check_diff()

test/pybind/rbd: don't ignore from_snapshot in check_diff()

Despite the test in test_diff_iterate() being correct, it started
failing:

    >       check_diff(self.image, 0, IMG_SIZE, 'snap1', [(0, 512, False)])
    ...
    a = [], b = [(0, 512, False)]
    ...
    >       assert a == b
    E       AssertionError

This is because check_diff() drops 'snap1' argument on the floor and
passes None to image.diff_iterate() instead.  This goes back to 2013,
see commit e88fe3cbbc8f ("rbd.py: add some missing functions").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit f8ced6d1fe667864bee224104c23b62a2c1c0bfc)