Fix for PyQgsVectorFileWriter test segfault on Mac

- Initialize newFilename to QString(), or QgsDebugMsg for newFilename segfaults (line 650)
- Set test assert to QgsVectorFileWriter::WriterError enum for success (NoError = 0, i.e. false)

tests/src/python/test_qgsvectorfilewriter.py

@@ -67,6 +67,7 @@ def testWrite(self):
         myLayerOptions = QStringList()
         mySelectedOnlyFlag = False
         mySkipAttributesFlag = False
+        myNewFileName = QString()
         myGeoCrs = QgsCoordinateReferenceSystem()
         myGeoCrs.createFromId(4326, QgsCoordinateReferenceSystem.EpsgCrsId)
         myResult = QgsVectorFileWriter.writeAsVectorFormat(
@@ -79,8 +80,9 @@ def testWrite(self):
             myErrorMessage,
             myOptions,
             myLayerOptions,
-            mySkipAttributesFlag)
-        assert myResult==True
+            mySkipAttributesFlag,
+            myNewFileName)
+        assert myResult==QgsVectorFileWriter.NoError
 
 if __name__ == '__main__':
     unittest.main()