Fix typos in TestThreadedProgressObservers tests

[pont-us]

test_threaded_progress_on_begin and test_threaded_progress_on_end were calling stop() on _patch rather than _mock_patch(), producing an error on the subsequent _mock_patch.start() (since it was already started, and hadn't been stopped). Corrected in this commit.

Fixes #1104

[Description of PR]

Checklist:

• [ ] Add unit tests and/or doctests in docstrings n/a
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions n/a
• [ ] New/modified features documented in docs/source/* n/a
• [ ] Changes documented in CHANGES.md n/a
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.58%. Comparing base (2c02ea2) to head (32dc1ed).
Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1105      +/-   ##
==========================================
- Coverage   89.94%   89.58%   -0.37%     
==========================================
  Files         276      277       +1     
  Lines       21573    21836     +263     
==========================================
+ Hits        19404    19561     +157     
- Misses       2169     2275     +106     

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