TST: generic surfaces

[Lucas-Prates]

Pull request type

• Code maintenance (refactoring, formatting, tests)

Checklist

• Tests for the changes have been added (if needed)
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

No tests.

New behavior

Unit tests for generic surfaces

Breaking change

• Yes
• No

Tasks

• Unit test for GenericSurfaces (97% coverage)
• Use tmpdir_factory to automatically clean up created CSV files
• Unit test for LinearGenericSurfaces

[MateusStano]

Everything seems good with the GenericSurfaces tests

I think we still need similar tests for LinearGenericSurfaces

The integration tests are low priority for now, so we can leave it for later if need

[Lucas-Prates]

Everything seems good with the GenericSurfaces tests

I think we still need similar tests for LinearGenericSurfaces

The integration tests are low priority for now, so we can leave it for later if need

Added tests for LinearGenericSurfaces with 100% coverage. I got an unexpected behavior of division by zero. I explained the issue in the #680 PR.