[CLN] Refactor test imports and file structure

Fixed import paths in `test_plot_3d.py` for `check_image_hash`. Moved `util_tests.py` under the appropriate `tests` directory for better organization.

Author: Leguark

tests/test_plotting/test_plot_3d.py

@@ -5,7 +5,7 @@
 import gempy as gp
 from gempy.core.data import GeoModel
 from gempy_viewer.core.scalar_data_type import TopographyDataType
-from test_plotting.util_tests import check_image_hash
+from tests.util_tests import check_image_hash
 from tests.conftest import _one_fault_model_generator
 
 np.random.seed(1234)