Refactored test and their fixtures.

nomad/processing/base.py

nomad/processing/data.py

nomad/processing/handler.py

tests/conftest.py

tests/processing/test_base.py

tests/processing/test_data.py

tests/test_api.py

tests/test_files.py

tests/test_normalizing.py

tests/test_parsing.py

tests/test_repo.py