fa06bb607da2d3e35723661156d873c8eac1fa50 qa: Do not force overwrite of QT_QPA_PLATFORM on windows for gui tests (MarcoFalke)
faccf5f9c899c40d4da5792629d0714249a4616b doc: Explain QT_QPA_PLATFORM for gui tests (MarcoFalke)
Pull request description:
Closes #17013
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Depends on D7748
Backport of Core PR17015