fix(sd_servers_with_designation_collisions): scope to individual test

---to avoid test-level collisions of the fixture itself when the test is
parameterized (e.g., for localization).

securedrop/tests/functional/test_source_designation_collision.py

@@ -7,7 +7,7 @@
 from tests.functional.conftest import spawn_sd_servers
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture
 def sd_servers_with_designation_collisions(setup_journalist_key_and_gpg_folder, setup_rqworker):
     """Spawn source and journalist apps that can only generate a single journalist designation."""
     # Generate a config that can only generate a single journalist designation