Migrate tests for model serialization to goldens.

This is to ensure that whenever we change the model serialization code, it is much easier to update the tests. The changes should be more readable this way.

When code changes to request changes to the goldens, the author of the change will need to run a `pytest` command to update the files by passing a flag, then add the goldens to the PR (ideally via a new commit).

This also ensures that we can test all models in all scenarios, in a similar way. Thus, removed the tests for empty models that were just testing for empty manifest. Added a new model that only contains an empty file.

Signed-off-by: Mihai Maruseac <[email protected]>

model_signing/serialization/conftest.py

@@ -19,23 +19,36 @@
 from model_signing.serialization import test_support
 
 
+def pytest_addoption(parser):
+    """Adds a flag argument to update the goldens."""
+    parser.addoption(
+        "--update_goldens",
+        action="store_true",
+        default=False,
+        help="update golden files",
+    )
+
+
 # Note: Don't make fixtures with global scope as we are altering the models!
 @pytest.fixture
 def sample_model_file(tmp_path_factory):
+    """A model with just a single file."""
     file = tmp_path_factory.mktemp("model") / "file"
     file.write_bytes(test_support.KNOWN_MODEL_TEXT)
     return file
 
 
 @pytest.fixture
 def empty_model_file(tmp_path_factory):
+    """A model with just an empty file."""
     file = tmp_path_factory.mktemp("model") / "file"
     file.write_bytes(b"")
     return file
 
 
 @pytest.fixture
 def sample_model_folder(tmp_path_factory):
+    """A model with multiple files and directories."""
     model_root = tmp_path_factory.mktemp("model") / "root"
     model_root.mkdir()
 
@@ -55,13 +68,27 @@ def sample_model_folder(tmp_path_factory):
 
 @pytest.fixture
 def empty_model_folder(tmp_path_factory):
+    """A model with just an empty directory."""
+    model_root = tmp_path_factory.mktemp("model") / "root"
+    model_root.mkdir()
+    return model_root
+
+
+@pytest.fixture
+def model_folder_with_empty_file(tmp_path_factory):
+    """A model with just an empty file, inside a directory."""
     model_root = tmp_path_factory.mktemp("model") / "root"
     model_root.mkdir()
+
+    empty_file = model_root / "empty_file"
+    empty_file.write_bytes(b"")
+
     return model_root
 
 
 @pytest.fixture
 def deep_model_folder(tmp_path_factory):
+    """A model with a deep directory hierarchy."""
     model_root = tmp_path_factory.mktemp("model") / "root"
     model_root.mkdir()