r.random.cells: migrate tests from gunittest to pytest

[Valyrian-Code]

Migrates the r.random.cells testsuite from gunittest to pytest, continuing the one-at-a-time migrations from #7635/#7675/#7780.

Keeps the same three cases and the same seed=100, so the reproduced values match the original test:

• distance=0.01 fills every cell with a unique category (cells == n, no nulls)
• distance=2.00001 (just over the cell width) fills less than half the cells; the exact count depends on the random sequence, so this keeps the original's loose upper-bound check rather than pinning an exact value
• ncells=12 limits the fill to exactly that many cells

Verified locally, all three pass.

[Copilot]

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Pull request overview

Migrates r.random.cells test coverage from the legacy gunittest harness to pytest, preserving the three existing behavioral cases and seed-based expectations.

Changes:

• Removes the gunittest-based test file under testsuite/.
• Adds a new pytest test module using an isolated GRASS project/session fixture.
• Re-implements the same three assertions using r.univar JSON output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
raster/r.random.cells/testsuite/test_random_cells.py	Removes the previous gunittest tests being migrated.
raster/r.random.cells/tests/r_random_cells_test.py	Adds pytest equivalents using grass.script + Tools and r.univar JSON assertions.

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[echoix]

@wenzeslaus Is there a known pattern for gunittest "assertRasterFitsUnivar" to pytest? Otherwise, we would be recopying the assertions but we are loosing the fact that these multiple independant asserts mean something when together (so the pattern will probably diverge accross files in the future).

[Valyrian-Code]

Thanks for the review. A few notes:

• Fixture scope: kept function-scoped on purpose, isolated per test with a fresh tmp_path project. This matches the pattern in the already-merged r.cross (r.cross: migrate tests from gunittest to pytest #7635) and r.null (r.null: migrate tests from gunittest to pytest #7675) migrations, and avoids the shared-state cleanup that a module or session scope would need between tests.
• SPDX-FileCopyrightText: neither r.null nor r.cross added this to their new pytest files, only SPDX-License-Identifier, so kept it consistent with that.
• Fixture name: "session" matches the name used in r.null and r.support's already-merged tests, so kept it for consistency across the migrated files.