Move test support to root of model_signing. (sigstore#252)

Since we will need to share some of the models with other directories
besides serialization, the only way to achieve this without duplication
is to have `conftest.py` in the root testing directory. And, when moving
that, we also need to move `test_support.py` which defines useful
constants used both in tests and in `conftest.py`.

Signed-off-by: Mihai Maruseac <mihaimaruseac@google.com>

model_signing/serialization/conftest.py → model_signing/conftest.py

@@ -16,7 +16,7 @@
 
 import pytest
 
-from model_signing.serialization import test_support
+from model_signing import test_support
 
 
 def pytest_addoption(parser):

model_signing/serialization/serialize_by_file_shard_test.py

@@ -12,6 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""Tests for shard serializers.
+
+NOTE: This test uses a golden setup to compute digest of several test
+models. If the golden tests are failing, regenerate the golden files with
+
+  pytest model_signing/ --update_goldens
+"""
+
 import dataclasses
 import pathlib
 import pytest
@@ -20,13 +28,7 @@
 from model_signing.hashing import memory
 from model_signing.manifest import manifest
 from model_signing.serialization import serialize_by_file_shard
-from model_signing.serialization import test_support
-
-
-# NOTE: This test uses a golden setup to compute digest of several test
-# models. If the golden tests are failing, regenerate the golden files with
-#
-#   pytest model_signing/serialization/ --update_goldens
+from model_signing import test_support
 
 
 class TestDigestSerializer:

model_signing/serialization/serialize_by_file_test.py

@@ -12,6 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""Tests for file serializers.
+
+NOTE: This test uses a goldens setup to compute digest of several test
+models. If the golden tests are failing, regenerate the golden files with
+
+  pytest model_signing/ --update_goldens
+"""
+
 import os
 import pathlib
 import pytest
@@ -20,13 +28,7 @@
 from model_signing.hashing import memory
 from model_signing.manifest import manifest
 from model_signing.serialization import serialize_by_file
-from model_signing.serialization import test_support
-
-
-# NOTE: This test uses a goldens setup to compute digest of several test
-# models. If the golden tests are failing, regenerate the golden files with
-#
-#   pytest model_signing/serialization/ --update_goldens
+from model_signing import test_support
 
 
 class TestDigestSerializer: