Refactor models tests

[Eirik Keilegavlen]

Tasks

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1. integration/models/test_ad_methods_single_phase_flow.py
2. integration/models/test_constitutive_laws.py
3. integration/models/test_diagnostics_mixin.py
4. integration/models/test_energy_balance.py
5. integration/models/test_equations.py
6. integration/models/test_evaluate_constitutive_laws.py
7. integration/models/test_geometry.py
8. integration/models/test_mass_balance.py
9. integration/models/test_materials_and_units.py
10. integration/models/test_momentum_balance.py
11. integration/models/test_poromechanics.py
12. integration/models/test_rediscretization.py
13. integration/models/test_restart.py
14. integration/models/test_thermoporomechanics.py
15. integration/models/test_variables.py
16. integration/models/test_well_models.py