Move all regression tests to pytests

**Is your feature request related to a problem? Please describe.**

Several of the existing regression tests simply do output diff checking. We should migrate them to python tests so we can do better assertions and print more clear error messages when tests fail

**Describe the solution you'd like**
Move all regression tests to pytests

Start by inventorying the existing regression tests and their output-diff checks. Review how those tests currently run, then identify the Python/pytest structure needed for migration. Done means all regression tests use Python tests with explicit assertions and clearer failure messages.

Written by the indexing model from the issue text.