Added tests for test submission files

tests/rest/test_submission_files.py

 import pytest
 
+from portal.database import ProjectConfig
 from tests.utils.ent_mocker import EntitiesMocker
 from . import utils
 
@@ -9,6 +10,8 @@ def mocked_submission(ent_mocker: EntitiesMocker, rest_service):
     submission = ent_mocker.create_submission()
     rest_service.submissions.write_entity(submission)
     ent_mocker.create_submission_storage(submission=submission)
+    project_config: ProjectConfig = submission.project.config
+    project_config.test_files_commit_hash = 'some-random-hash'
     return submission
 
 
@@ -31,3 +34,10 @@ def test_submission_results_are_available(client, mocked_submission):
     assert response.data
     assert response.data.decode('utf-8') == expected('student.json')
 
+
+def test_submission_test_files_are_available(client, mocked_submission):
+    s = mocked_submission
+    response = utils.make_request(client, f'/submissions/{s.id}/files/test_files?path=test_main.c')
+    assert response.status_code == 200
+    assert response.data
+    assert response.data.decode('utf-8') == expected('test_main.c')