Add tests for the names of migration files

[ewjoachim]

Closes #251

Successful PR Checklist:

• [x] Tests
  • [ ] (not applicable?)
• [ ] Documentation
  • [x] (not applicable?)

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• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20feature%20%E2%AD%90%EF%B8%8F
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