Fetch `ManifestV1` from fractal-server in tests (close #537)

[tcompa]

Checklist before merging

• [ ] I added an appropriate entry to CHANGELOG.md N/A

[github-actions[bot]]

Coverage report

The coverage rate went from 89.8% to 89.8% :arrow_down: The branch rate is 83%.

None of the new lines are part of the tested code. Therefore, there is no coverage data about them.