[test] Test periodic subgrids

What this MR does / why does DuMux need it:

This extends the test suite to further cover subgrids of periodic SP host grids.

• The test of the subgrid gridmanager is extended to test periodicity detection of the wrapped grid.
• Adds a freeflow test in test/freeflow/navierstokes/periodic/porescale with geometry generated using createGridFromImage() from a .pbm file and tiles this periodic geometry into a larger domain.

Notes for the reviewer

With !3888 (merged) DuMux now supports periodic SubGrids as long as the host grid's periodicity is supported.

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• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
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• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
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