macOS 11 machines don't allow tabbing to buttons

On macOS, tabbing between controls does by default not include controls that don't accept text input. Ie you can tab between line edits, but not to push buttons.

The Keyboard preferences have a setting for this under Keyboard/Shortcuts/"Use keyboard navigation to move focus between controls". This checkbox needs to be checked, otherwise tests that simulate focus handling using tab-navigation fail on those platforms.

As per the test run https://testresults.qt.io/coin/integration/qt/qtbase/tasks/1637088112, the Intel macOS 11 machine passed the test, but the ARM machine failed the test, suggesting that the former is provisioned correctly, whereas the latter is not. Correction: the macOS 11 Intel machines on which the test run passed was a MinimalStaticTests configuration, and didn't run the Widget test at all.