tests: re-enable test_unbuffered_stdio on CI

The contemporary versions of pytest-xdist and execnet are said to have
sorted out the issue with tests sometimes not running in the main
thread, so we should be able to run this test on our CI now.

tests/functional/test_unbuffered_stdio.py

@@ -20,10 +20,6 @@
 from PyInstaller.compat import is_win
 
 
-@pytest.mark.skipif(
-    os.environ.get('CI', 'false').lower() == 'true',
-    reason="The test does not support CI (pytest-xdist sometimes runs it in secondary thread)."
-)
 @pytest.mark.parametrize('stream_mode', ['binary', 'text'])
 @pytest.mark.parametrize('output_stream', ['stdout', 'stderr'])
 def test_unbuffered_stdio(tmp_path, output_stream, stream_mode, pyi_builder_spec):