tst_QWindow::initialSize: Use showNormal() instead of show()

On some platforms show() may result in the window being shown full
screen, depending on the defaultWindowState of the platform. If
that's the case, the resulting window size does not follow
the requested size (though the internal normal geometry should).

We need to use showNormal, so that we guarantee that the window
state is WindowNoState.

Change-Id: Ied080eaca13d759501ea292b5e6b2df4e3b32426
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>