GUI: Tests

[AhmetCanSolak]

This PR introduces tests on the GUI related code pieces. Might not cover everything within this PR but I will try to test the particular pain points at which we seem to fail detecting the breaking changes.

[royerloic]

Closing — Stale, concept documented for future work

Thank you @AhmetCanSolak — GUI testing with pytest-qt is an important initiative.

Why closing:

• Uses old requirements/development.txt (now pyproject.toml [dev] extras)
• Only contains one test (test_activity_widget)
• Has a bug: self.infoTextBox no-op statement on line 38

What's valuable and preserved:

• The pytest-qt testing pattern is documented in code_review_reports/pr_ideas_for_future_work.md
• The MatplotlibWidget and ActivityWidget test patterns serve as templates for future GUI test development