Add skyCatalog test for object position test.

[cwwalter]

Mike notes that currently, test_object_positions.py only is testing that things work with instance catalogs and that we should add tests for the skyCatalog case too.

Currently, he notes we are only checking if we can run a test in the sky catalogs by running the imsim-user-skycat.yaml file.

This issue is to figure out and then implement when we need to also make specific tests for all the different input cases.

[jchiang87]

The original motivation for the test_object_positions.py test was that we were finding that the rendered positions of stars didn't match the catalog positions for the LsstCamImSim instrument when using batoid ray-tracing. Ultimately, the cause of the problem was that the camera wasn't being specified consistently everywhere, i.e., the photon_ops code was always using LsstCam, even if LsstCamImSim was set in the config. In any case, the source of the object positions was not really relevant.

Re-running using a skycat input would test code that probably isn't being tested currently, but I think it would be better to write tests that address those code paths directly, e.g., tests that exercise InstCatWorldPos and SkyCatWorldPos, because that code (and what happens upstream) would be the relevant differences here.