New `modified_dataset` fixture

[mih]

Promote a previously internal fixture to provide a standard modified_dataset fixture. This fixture is sessions-scope, and yields a dataset with many facets of modification, suitable for testing change reporting. The fixture verifies that no modifications have been applied to the testbed.

Extracted from #588

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3d0c8a4) 92.99% compared to head (c2ad5b8) 92.99%. Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #612 +/- ## ======================================= Coverage 92.99% 92.99% ======================================= Files 160 160 Lines 11863 11864 +1 Branches 1795 1795 ======================================= + Hits 11032 11033 +1 - Misses 640 642 +2 + Partials 191 189 -2 ```

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