testlib: Disable window restoration on macOS in a non-persistent way

The recommended way to register defaults on macOS is via registerDefaults:
which puts the key/value into the volatile NSRegistrationDomain.

This stops testlib from dumping preference files into ~/Library/Preferences:

  ❯ plutil -p tst_qwidget.plist
  {
    "ApplePersistenceIgnoreState" => 1
  }

Pick-to: 6.2 6.3 5.15
Change-Id: I4ac812014f9ff97bc446806eaf2108cd1c8b89c3
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/testlib/qtestutil_macos.mm

@@ -53,8 +53,11 @@ namespace QTestPrivate {
         to start with a clean slate and prevents the "previous restore failed"
         dialog from showing if there was a test crash.
     */
-    void disableWindowRestore() {
-        [[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"ApplePersistenceIgnoreState"];
+    void disableWindowRestore()
+    {
+        [NSUserDefaults.standardUserDefaults registerDefaults:@{
+            @"ApplePersistenceIgnoreState" : @YES
+        }];
     }
 
     bool macCrashReporterWillShowDialog()