QuickTest runs test suites in parallel, causing crashes

If a single qml file has multiple TestCase elements, all of which have when-properties that evaluate to true, we might end up in a situation where multiple TestCase elements run their test functions at the same time. I don't how this can happen, as my understanding is that the QML engine is single threaded.

But it happens, and that's bad, as Qt Testlib, which Qt Quick Test uses for reporting, does not expect test functions to be executed in parallel.

WIP patch at https://codereview.qt-project.org/c/qt/qtdeclarative/+/372593