Add coverage for QT_ALLOW_DOWNLOAD configure option

The configure system of Qt features the QT_ALLOW_DOWNLOAD flag. We should add coverage for this in some CI configurations for Qt.

Where is Qt_ALLOW_DOWNLOAD used?

Currently, QT_ALLOW_DOWNLOAD is used to enable some tests and examples:

• qtbase/tests/manual/iconbrowser/CMakeLists.txt
• qtdoc/examples/demos/CMakeLists.txt
• qtgrpc/tests/auto/grpc/client/shared/CMakeLists.txt
• qtgrpc/tests/auto/protobuf/conformance/CMakeLists.txt

Which configurations should test for this?

It would be good to combine -DQT_ALLOW_DOWNLOAD=ON with some configurations that build tests and examples (see above).

Note that qtgrpc already does have infrastructure to enable the option locally in some configurations: https://codereview.qt-project.org/c/qt/qtgrpc/+/544463/9/coin/module_config.yaml . But it seems that a matching configuration was never added.