add a unit test for multi-cell SDM adaptivity covering the scenarion from adaptivity paper diagram (attached)

[slayoo]

[slayoo]

Likely place where it should go: https://github.com/open-atmos/PySDM/blob/main/tests/unit_tests/dynamics/collisions/test_sdm_multi_cell.py

[slayoo]

starting point: https://github.com/open-atmos/PySDM/blob/main/tests/unit_tests/dynamics/collisions/test_sdm_single_cell.py (see gamma-mocking logic)

[slayoo]

• [ ] turnb off the adaptivity and assert that deficit occurs

[github-actions[bot]]

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