Tests halt with PySide 6.7.1 on cp311 (Linux)

Tests halt on rocky linux on Python 3.11 with PySide6 6.7.1 and one needs to kill the process. From the output the issue is during traitsui.testing.tests.test_exception_handling.TestExceptionHandling.test_error_from_gui_captured_and_raise

```
[2024-06-12T15:33:11.186Z] b'test_commands_imports (traitsui.testing.tests.test_api.TestApi.test_commands_imports) ... ok'
[2024-06-12T15:33:11.186Z] b'test_exceptions_imports (traitsui.testing.tests.test_api.TestApi.test_exceptions_imports) ... ok'
[2024-06-12T15:33:11.186Z] b'test_locator_imports (traitsui.testing.tests.test_api.TestApi.test_locator_imports) ... ok'
[2024-06-12T15:33:11.187Z] b'test_query_imports (traitsui.testing.tests.test_api.TestApi.test_query_imports) ... ok'
[2024-06-12T15:33:11.187Z] b'test_tester_import (traitsui.testing.tests.test_api.TestApi.test_tester_import) ... ok'
[2024-06-12T15:33:11.187Z] b'test_error_from_gui_captured_and_raise (traitsui.testing.tests.test_exception_handling.TestExceptionHandling.test_error_from_gui_captured_and_raise) ... FAIL'
[2024-06-12T15:33:11.187Z] b''
[2024-06-12T15:33:11.187Z] b'======================================================================'
[2024-06-12T15:33:11.187Z] b'FAIL: test2_test_pyside6 (__main__.Tests.test2_test_pyside6)'
[2024-06-12T15:33:11.187Z] b'----------------------------------------------------------------------'
[2024-06-12T15:33:11.187Z] b'Traceback (most recent call last):'
[2024-06-12T15:33:11.187Z] b' File "/home/builder/workspace/Buildsystem/test/new/cp311/traitsui-8.0.0/tests/run.py", line 36, in test2_test_pyside6'
[2024-06-12T15:33:11.187Z] b' self.runpy('
[2024-06-12T15:33:11.187Z] b' File "/home/builder/workspace/Buildsystem/test/new/cp311/entest_utils.py", line 469, in runpy'
[2024-06-12T15:33:11.187Z] b' self.run_test_function('
[2024-06-12T15:33:11.187Z] b' File "/home/builder/workspace/Buildsystem/test/new/cp311/entest_utils.py", line 546, in run_test_function'
[2024-06-12T15:33:11.187Z] b' self.fail("unexpected test exit code {!r}".format(ret))'
[2024-06-12T15:33:11.187Z] b'AssertionError: unexpected test exit code -15'
[2024-06-12T15:33:11.187Z] b''
[2024-06-12T15:33:11.187Z] b'----------------------------------------------------------------------'
[2024-06-12T15:33:11.187Z] b'Ran 18 tests in 1189.950s'
[2024-06-12T15:33:11.187Z] b'FAILED (failures=1, skipped=11)'
[2024-06-12T15:33:11.187Z] b'==================================================='
[2024-06-12T15:33:11.187Z] b"ENTEST STOP -- TOTAL FAILURES: 1 ('traitsui-8.0.0')"
[2024-06-12T15:33:11.187Z] b'==================================================='
```

Start with tests/run.py and reproduce test2_test_pyside6 on Rocky Linux with Python 3.11 and PySide6 6.7.1. Focus on traitsui.testing.tests.test_exception_handling.TestExceptionHandling.test_error_from_gui_captured_and_raise and compare the reported failure and process halt. Done means the test run completes without requiring the process to be killed and the test no longer fails with exit code -15.

Written by the indexing model from the issue text.