Instead of an unconditional wait.
The QSKIP is necessary because the PDF format is non-native and the Windows
implementation of QPrintDialog doesn't work then.
The macOS implementation has the same warning, so add the check for that OS
as well.
The test has previously passed, since it hasn't seen not actually opening the
dialog as an issue.
Change-Id: Ib53e378b232580358f13e0c5206404412f9e6ee2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>