Split schema tests into multiple files.

tests/schema/__init__.py

tests/schema/snapshots/__init__.py

tests/schema/snapshots/snap_test_authors.py

tests/schema/snapshots/snap_test_login_shortcuts.py

tests/schema/snapshots/snap_test_node.py

tests/schema/test_authors.py

tests/schema/test_login_shortcuts.py

tests/schema/test_node.py

tests/test_schema.py