qtdiag: Disable autotest for all cross-built platforms

Timeouts have been observed on
arm-little_endian-ilp32-eabi-hardfloat shared (dynamic).

Change-Id: I6e6525c961e8b2611a8f5485175a42a83facd723
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>