TST Refactor unittest to pytest style custom tests (#2573)

This is a follow up to #2462, #2478, and #2491.

Finish the refactor from unittest-style tests to pytest-style tests to
now also include the last big file to still use the old style,
test_custom_models.py. This file was already mostly written with pytest
in mind, so the changes were rather minimal.

With this class refactored, we can finally remove ClassInstantier, which
made understanding test parametrization much more difficult.

Author: BenjaminBossan