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 (26634d6) to head (71753f7).
Report is 62 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              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[alejoe91]

Suggested change

	from ..utilities import validate_probe_dict
	from probeinterface.testing import validate_probe_dict

@musicinmybrain can you elaborate on why it's not a good ide to include the validate_probe_dict in the API? I think it's an important function also for the package, so I would leave it there...

[alejoe91]

Same here

[alejoe91]

@musicinmybrain thanks for working on this!
After some discussion, we merged this one instead #326, that adds the schema to the package data and makes jsonschema import optional