The minimum CMake version for Qt6 is 3.16, so all the calls to
cmake_minimum_required() are updated here to enforce that
minimum. This will avoid any CMake version-related warnings
from Qt.
Avoid hard-coding Qt5 where the tests could now be using
Qt5 or Qt6.
Fixes: #22188
In order to support modern macOS features like Dark Mode, we need to use
Qt 5.15, which requires macOS 10.13. However, we still want to support
macOS 10.10 as well, for which we need to use Qt 5.9. Build separate
macOS packages for these use cases.
Fixes: #21606
Issue: #20825
Also add comments for sections to make it easier to figure out what's
going on.
Also rename the `cmake_test_unix_package` to be Linux-specific since it
actually is.