Add Unit Testing Workflow using PyTest

[DavidoTek]

Fix https://github.com/DavidoTek/ProtonUp-Qt/issues/347

• Adds a CI workflow for Unit Testing using pytest
  • Runs on Pull Requests and Pushes to main
  • Currently tests on Ubuntu 22.04 and Python 3.10
• Adds an initial test for util.py#get_random_game_name
• Produces Markdown Test Report using pytest-md

---

Fixed Qt import for PyTest by installing libEGL and libxcbcommon and setting QT_QPA_PLATFORM=offscreen

**Previously**: Currently fails due to missing `libEGL`, i.e. the runner is headless but the Qt imports expect a graphics library. I will investigate that. ```sql ==================================== ERRORS ==================================== _____________________ ERROR collecting tests/test_util.py ______________________ ImportError while importing test module '/home/runner/work/ProtonUp-Qt/ProtonUp-Qt/tests/test_util.py'. Hint: make sure your test modules/packages have valid Python names. Traceback: /opt/hostedtoolcache/Python/3.10.14/x64/lib/python3.10/importlib/__init__.py:126: in import_module return _bootstrap._gcd_import(name[level:], package, level) tests/test_util.py:1: in from pupgui2.util import * pupgui2/util.py:20: in from PySide6.QtWidgets import QApplication, QStyleFactory, QMessageBox, QCheckBox E ImportError: libEGL.so.1: cannot open shared object file: No such file or directory ---------------- generated Markdown report: /tmp/tmp.t8xQFKNH8p ---------------- ```