new unit tests for compile_cost_assumptions.py

[finozzifa]

Closes # (if applicable).

Changes proposed in this Pull Request

The pull request introduces unit tests for the compile_cost_assumptions.py functions:

• get_dea_vehicle_data
• rename_pypsa_old

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Data source for new technologies is clearly stated.
• Newly introduced dependencies are added to environment.yaml (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the GPLv3 license.