Split fixtures from conftest

For better management, extract fixtures from tests/conftest.py to appropriate files in tests/fixtures/ and import them via pytest_plugins. Conftest should not directly import those modules too.

The remaining fixtures were left in conftest for now. Some might go into fixtures/infrastructure.py.

There are still a lot of fixtures in fixtures/data.py. Maybe they could be split further by type of data later.

Uploads connected to groups were moved to fixtures/groups.py. When groups and users have been merged, those uploads may be moved/merged together.