Verify that the temporary directory could be created. Check whether
the gtk-update-icon-cache binary exists before running and skip cleanly.
Check successful execution. Fixes Windows warnings:

SKIP   : tst_QIcon::fromThemeCache() gtk-update-icon-cache not run
.\tst_qicon.cpp(707) : failure location
QWARN  : tst_QIcon::fromThemeCache() QTemporaryDir: Unable to remove "D:\\temp\\tst_qicon-DSSn9G" most likely due to the presence of read-only
files.

Change-Id: Ibc8f883121e62b30d71586bc64b42eb6c480925f
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>