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Sometimes we "forget" about the slow tests for weeks and we get outdated tests.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

• [x] No

Additional information

Enter text here...

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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