Ensure proper cleanup and error handling in test fixtures

[rodja]

Motivation

If an exception happens inside the fixture setup, the fixture was not reset correctly leading to failed tests.

Implementation

• Updated screen and user fixtures to use try...finally for resource cleanup, ensuring environment variables are reset and servers are stopped even if an error occurs.
• Improved error logging by checking for unexpected ERROR logs after yielding the fixture.
• reactivate tests from test_main_file_marker.py (which failed before)

This change enhances the reliability of the testing framework by ensuring that resources are properly managed and that errors are logged appropriately.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been reactivated.
• Documentation is not necessary.

[evnchn]

This is good. Should address the FEFEFEFEFEFE which occurs in the tests when the headless Chromium did not get shutdowned by the last tests which failed in an unclean manner (port occupied thereafter), which definitely did occur at least once among https://github.com/evnchn/nicegui/actions/workflows/test.yml but I can't quite find it.