Reduce warnings in tests

[heikoklein]

This issue is a long term issue to address warnings appearing in the code, i.e. caused by deprecations. Checklist:

• In pyproject.toml turn warnings into errors, i.e. uncomment filterwarnings section https://github.com/metno/pyaerocom/blob/d714b101e1ce52f14b22a0b51b2ec4b8e09bfaf6/pyproject.toml#L105-L117
• Turn all errors into new issues to be done until next milestone
• Move this issue to a issue 2 milestones/month ahead.