The previous sync() of the specific scope and domain may have failed due
to not having the necessary permissions, but the saved value may still be
cached, so we need to check both.
This was observed on macOS Sierra, where a failed sync() will result in
marking the CFPrefsPlistSource as read-only, eg:
2016-10-04 13:14:11.713271 tst_qsettings[88537:767733] [User Defaults] attempt to set
<private> for key in <private> in read-only (due to a previous failed write) preferences
domain CFPrefsPlistSource<0x6180000e1780> (Domain: org.software.KillerAPP,
User: kCFPreferencesAnyUser, ByHost: Yes, Container: (null))
Change-Id: I8976c1c4acfe2cb0d5510298d5c585faca9607f6
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>
76f53791a1