Investigate and (hopefully) resolve segfaults and other issues in UIC tests

[CAM-Gerlach]

Originally discussed in PR #262 .

At present, a number of the UIC tests are skipped for various reasons, which might need to be investigated. Most prominently, the tests segfault on our Linux CIs (unconfirmed if this also occurs locally) under all Qt5 and Qt6 bindings. Furthermore, test_load_ui_type fails on Pyside2/Pyside6 on most platforms, due to what looks like missing Pyside2uic, and oddly, test_load_custom_ui_auto fails on (aside from Linux CIs) one very specific platform: Python 3.9 + macOS + pip with a error message that seems like it might be solvable (here's the check run. Perhaps at least some of these issues can be investigated and eliminated, preferably by someone wiser than I about Qt.