Cleanup testing: refactor and standardize. (#237)

* Cleanup testing: refactor and standardize.

Rename `fixtures_constants` to `test_support` as it will also contain test utilities that can be used to make tests more readable.

Tests should now follow a similar structure (test file, test model changes, test folder, test deep folder, test empty, test special), with similar test names and similar spacing between the lines (in general arrange-act-assert, except for tests such as `test_model_with_empty_folder_hashes_differently_than_with_empty_file` which need to create an empty dir, compute a digest, delete the dir, create an empty file, compute a new digest, compare the 2 digests). Some tests are not duplicated in the other class as they don't make sense (e.g. itemized serializers produce no result on empty folders). Some tests have slightly different names in the other class due to behavior differences (e.g., corner cases around empty files).

Remove the `test_special_file` that tests for FIFOs in every test suite as we only need to test it once (this way, the special way this test is written is not replicated multiple times).

The code refactoring in the next PR will also remove some of the corner cases around these tests.

Since Critique gets confused by lines changed in the PR (as tests got reordered), it might be better to just review the file in its entirety

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

* Remove unused imports

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

* Fix line length

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

* Simplify tests

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

---------

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

model_signing/serialization/fixtures.py

@@ -16,14 +16,14 @@
 
 import pytest
 
-from model_signing.serialization import fixtures_constants
+from model_signing.serialization import test_support
 
 
 # Note: Don't make fixtures with global scope as we are altering the models!
 @pytest.fixture
 def sample_model_file(tmp_path_factory):
     file = tmp_path_factory.mktemp("model") / "file"
-    file.write_bytes(fixtures_constants.KNOWN_MODEL_TEXT)
+    file.write_bytes(test_support.KNOWN_MODEL_TEXT)
     return file