samples: convert pytest script to Python library

[mniestroj]

All the Python library interfaces are complete in terms of requirements for pytest testing. This makes it a perfect time to covert all existing pytest test cases to actually use that library instead of executing goliothctl in a subprocess. This gives much more flexibility on filtering data and timeout as well al error handling.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 21b361c):

https://golioth-zephyr-sdk-doxygen-dev--pr368-samples-drop-gol-tjk974ro.web.app

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