Fix references to testing conf

tests/test_model.py

 import unittest
-from conf.testing import czifile
+from model_server.conf.testing import czifile
 from model_server.base.accessors import CziImageFileAccessor
 from model_server.base.models import DummySemanticSegmentationModel, DummyInstanceSegmentationModel, CouldNotLoadModelError
 

tests/test_workflow.py

 import unittest
 
-from conf.testing import czifile, output_path
+from model_server.conf.testing import czifile, output_path
 from model_server.base.models import DummySemanticSegmentationModel
 from model_server.base.workflows import classify_pixels