DEV: adds scheduled test with pytest slow option

[Gui-FernandesBR]

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)
• ReadMe, Docs and GitHub updates
• Other (please describe):

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

Sometimes we "forget" about the slow tests for weeks and we get outdated tests.

New behavior

Every 2 weeks, the slow tests will be executed on GitHub Actions

Breaking change

• No

Additional information

Enter text here...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.70%. Comparing base (3130cdb) to head (655f4be).
Report is 84 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #639      +/-   ##
===========================================
+ Coverage    73.89%   75.70%   +1.81%     
===========================================
  Files           79       81       +2     
  Lines        10073     9813     -260     
===========================================
- Hits          7443     7429      -14     
+ Misses        2630     2384     -246     

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