Move probeinterface.testing out of the API; fixes #321

[musicinmybrain]

This is one idea for how to deal with #321.

$ python3 -m venv _e
$ . _e/bin/activate
(_e) $ pip install -e .[test]
(_e) $ python3 -m pytest -v
[…]
= 86 passed, 1 warning in 2.69s =

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.53%. Comparing base (bf67e88) to head (71753f7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
- Coverage   89.58%   89.53%   -0.05%     
==========================================
  Files          12       11       -1     
  Lines        1939     1930       -9     
==========================================
- Hits         1737     1728       -9     
  Misses        202      202              

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