Migrate to pytest.

[jetuk]

We've started converting the existing unittests to pytest with fixtures. I've opened this issue so we can track which modules have been done and which are left to complete.

• [ ] test_attributes.py
• [ ] test_concurrency.py
• [x] test_data.py
• [ ] test_groups.py
• [ ] test_image.py
• [ ] test_load.py
• [ ] test_network.py
• [ ] test_notes.py
• [ ] test_permissions.py
• [ ] test_plugins.py
• [ ] test_project.py
• [ ] test_rules.py
• [ ] test_scenario.py
• [x] test_templates.py
• [ ] test_timeseries.py
• [ ] test_units.py
• [x] test_users.py

[jetuk]

Tag #11 for migrating attribute tests.