tst_QImageWriter: skip writing unsupported image formats

Do not try to test writing of image formats which are not supported on
the platform under testing. This test was failing for .ico files on
webOS.

Fixes: QTQAINFRA-4722
Pick-to: 6.3
Change-Id: Ic7d40d9751e0f921d9721c01492cca28d798e141
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

tests/auto/gui/image/qimagewriter/tst_qimagewriter.cpp

@@ -281,6 +281,8 @@ void tst_QImageWriter::writeImage2()
     QFETCH(QByteArray, format);
     QFETCH(QImage, image);
 
+    SKIP_IF_UNSUPPORTED(format);
+
     //we reduce the scope of writer so that it closes the associated file
     // and QFile::remove can actually work
     {