docs: never run test binaries without the offscreen platform

tests/CMakeLists.txt sets QT_QPA_PLATFORM=offscreen for ctest only, so a
binary invoked directly inherits the desktop's setting and throws real
windows onto the user's screen. Each test function builds its own
MainWindow, so one direct run of test_mainwindow flashes over a hundred
windows. Launching the application unasked is the same problem: running
it is a hand test and belongs to the user.

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