Re-work extension point test mocking

[cottsay]

The existing mocks are fragile and don't reliably work in all cases.

This approach uses an actual Distribution instance so that we don't need to modify it to reflect upstream changes to the interface.

[codecov[bot]]

Codecov Report

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

Comparison is base (ee44262) 83.37% compared to head (df3deec) 83.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #622 +/- ## ======================================= Coverage 83.37% 83.37% ======================================= Files 66 66 Lines 3789 3789 Branches 737 737 ======================================= Hits 3159 3159 Misses 556 556 Partials 74 74 ```

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