MNT: fix pylint errors in the tests module

[Gui-FernandesBR]

Pull request type

• Other (please describe): Tests and linters

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• 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

I didn't pylint the tests module earlier because @lucasfourier was refactoring the tests. Now I can finally apply pylint for tests!

New behavior

Now we are going to see pylint and flake8 warning in the tests module as well!

Breaking change

• No

Additional information

• Please approves MNT: Refactors the code to adopt flake8 #631 first.
• It is much easier to refactor code when it is not a part of the public API...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.85%. Comparing base (fd4941c) to head (44cd770).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #632      +/-   ##
===========================================
- Coverage    73.92%   73.85%   -0.08%     
===========================================
  Files           70       70              
  Lines        10032    10035       +3     
===========================================
- Hits          7416     7411       -5     
- Misses        2616     2624       +8     

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[MateusStano]

Seems good

I really like the grouping of tests into Classes