test: use flatter import

tests/test_active_learning.py

tests/test_bayes_inference.py

tests/test_latin.py

tests/test_mass_point_model.py

tests/test_metropolis_hastings.py

tests/test_ravaflow24.py

tests/test_robustgasp.py

tests/test_run_mass_point_model.py

tests/test_run_ravaflow24.py

tests/test_run_simulator.py

tests/test_saltelli.py

tests/test_sobol.py

tests/test_util_funcs.py