test: add PSS/E dynamic simulation regression coverage

[BurhanAbdullah]

Summary

• add end-to-end PSS/E dynamic simulation regression coverage
• exercise the dynamic workflow from solved case through model loading, channel setup, initialization, and a 1 s run
• parse the generated legacy PSS/E output with dyntools.CHNF and validate the time series and expected channels
• automatically skip when PSS/E or the validation case is unavailable

Validation

• PSS/E Xplore 36.7.0
• Python 3.14.6
• py -3.14 -m pytest PSSE/tests/test_dynamic_regression.py -v -s
• 1 passed in 5.88s
• diff limited to PSSE/tests/test_dynamic_regression.py