Speed up unittests

I'm a bit annoyed by the long time of our unittests, especially on TeamCity.
The from_imod5_data tests we have seem to be very slow. Probably worth some investigation.
Running our unittests locally with --duration=50 prints:

75.13s call     imod/tests/test_mf6/test_mf6_wel.py::test_import_and_convert_to_mf6[Well]
65.38s call     imod/tests/test_mf6/test_mf6_hfb.py::test_hfb_from_imod5
59.48s call     imod/tests/test_mf6/test_mf6_drn.py::test_from_imod5[default]
58.78s call     imod/tests/test_mf6/test_mf6_regrid_model.py::test_model_regridding_can_skip_validation
58.78s call     imod/tests/test_mf6/test_mf6_riv.py::test_import_river_from_imod5
58.02s call     imod/tests/test_mf6/test_mf6_wel.py::test_import_and_convert_to_mf6[LayeredWell]
49.52s call     imod/tests/test_mf6/test_mf6_drn.py::test_from_imod5[custom]
48.75s call     imod/tests/test_mf6/test_mf6_riv.py::test_import_river_from_imod5__negative_layer
48.09s call     imod/tests/test_mf6/test_mf6_regrid_model.py::test_model_regridding_can_validate
47.94s call     imod/tests/test_mf6/test_mf6_chd.py::test_from_imod5
47.69s call     imod/tests/test_mf6/test_mf6_chd.py::test_from_imod5_shd
42.67s setup    imod/tests/test_mf6/test_mf6_rch.py::test_planar_rch_from_imod5_constant
39.42s call     imod/tests/test_mf6/test_mf6_drn.py::test_from_imod5__negative_layer
38.98s call     imod/tests/test_mf6/test_mf6_riv.py::test_import_river_from_imod5__infiltration_factors
38.24s call     imod/tests/test_mf6/test_mf6_riv.py::test_import_river_from_imod5__period_data
37.24s call     imod/tests/test_mf6/test_mf6_generalheadboundary.py::test_from_imod5_planar
37.21s call     imod/tests/test_mf6/test_mf6_generalheadboundary.py::test_from_imod5_non_planar
35.37s setup    imod/tests/test_mf6/test_mf6_wel.py::test_import_and_convert_to_mf6[Well]
31.77s call     imod/tests/test_mf6/test_mf6_rch.py::test_planar_rch_from_imod5_transient
31.03s call     imod/tests/test_mf6/test_mf6_rch.py::test_planar_rch_from_imod5_constant
30.82s call     imod/tests/test_mf6/test_mf6_rch.py::test_non_planar_rch_from_imod5_constant
29.59s setup    imod/tests/test_mf6/test_mf6_chd.py::test_from_imod5
28.04s call     imod/tests/test_mf6/test_mf6_dis.py::test_from_imod5_data__validation_error
24.86s call     imod/tests/test_mf6/test_mf6_wel.py::test_import_and_cleanup[Well]
23.19s call     imod/tests/test_mf6/test_mf6_rch.py::test_non_planar_rch_from_imod5_transient
23.03s call     imod/tests/test_mf6/test_mf6_dis.py::test_from_imod5_data__write
22.72s call     imod/tests/test_mf6/test_mf6_dis.py::test_from_imod5_data__idomain_values
22.11s call     imod/tests/test_mf6/test_mf6_dis.py::test_from_imod5_data__grid_extent
22.06s call     imod/tests/test_mf6/test_mf6_rch.py::test_from_imod5_cap_data
14.98s setup    imod/tests/test_mf6/test_mf6_sto.py::test_from_imod5
11.26s call     imod/tests/test_mf6/test_multimodel/test_mf6_partitioning_unstructured.py::test_partition_transport_multispecies[three_parts]
10.46s call     imod/tests/test_mf6/test_mf6_mask_simulation.py::test_mask_simulation
10.35s setup    imod/tests/test_mf6/test_mf6_drn.py::test_from_imod5[default]
10.00s setup    imod/tests/test_mf6/test_mf6_hfb.py::test_hfb_from_imod5
9.40s call     imod/tests/test_mf6/test_mf6_regrid_transport.py::test_regrid_transport[col_row_dimension2]
9.19s setup    imod/tests/test_mf6/test_mf6_riv.py::test_import_river_from_imod5
8.97s call     imod/tests/test_mf6/test_multimodel/test_mf6_partitioning_unstructured.py::test_partition_transport_multispecies[two_parts]
8.76s call     imod/tests/test_mf6/test_multimodel/test_mf6_partitioning_unstructured.py::test_partition_transport[three_parts]
8.33s call     imod/tests/test_mf6/test_multimodel/test_mf6_modelsplitter_transport.py::test_split_flow_and_transport_model_evaluate_output_with_species
7.86s call     imod/tests/test_mf6/test_multimodel/test_mf6_modelsplitter_transport.py::test_split_flow_and_transport_model_evaluate_output
7.31s call     imod/tests/test_mf6/test_multimodel/test_mf6_partitioning_unstructured.py::test_partition_transport_multispecies[concentric]
7.22s call     imod/tests/test_mf6/test_mf6_regrid_transport.py::test_regrid_transport[col_row_dimension0]
6.41s setup    imod/tests/test_mf6/test_mf6_ic.py::test_from_imod5
6.35s call     imod/tests/test_mf6/test_multimodel/test_mf6_modelsplitter_transport.py::test_split_dump
6.07s call     imod/tests/test_mf6/test_multimodel/test_mf6_modelsplitter_transport.py::test_split_flow_and_transport_model
6.03s setup    imod/tests/test_mf6/test_mf6_generalheadboundary.py::test_from_imod5_non_planar
5.99s call     imod/tests/test_mf6/test_mf6_regrid_transport.py::test_regrid_transport[col_row_dimension1]
5.88s setup    imod/tests/test_mf6/test_mf6_riv.py::test_import_river_from_imod5__period_data
5.50s setup    imod/tests/test_mf6/test_mf6_dis.py::test_from_imod5_data__idomain_values
5.46s call     imod/tests/test_mf6/test_multimodel/test_mf6_partitioning_unstructured.py::test_partition_transport[concentric]

Start by running the unittest suite with --duration=50 and inspect the listed slow tests under imod/tests/test_mf6/, especially the from_imod5 tests and regridding tests. Compare setup and call times to identify where investigation should focus; done means the relevant unittest runtime is reduced and the affected tests still pass.