I think this line was placed into the test specifically because the original implementation of `all_world2pix` was not working when WCS did not contain non-linear distortions (see #2294) so it was there to make sure that the test does not crash because if test image had only a "simple" WCS.

However, `all_world2pix` **should** work for images without non-linear distortions (and there is no reason why a numerical method should not be able to solve an equation such as `y=2x+1`).

Now, if one accepts that `all_world2pix` must work for any WCS (be it with or without SIP), what is the point of having this assertion??? That is, what one really wants to check is that he/she really gets a correct inverse. `assert np.any(all_pix != wcs_pix)` does not provide any information.

For example, (just for fun), try the existing test on an image without SIP...