[tests] Fix MockSyncItem key assignment

Detected by CommandHandlerTest::testSyncAdd(),
CommandHandlerTest.cpp:286

The test case has most probably started failing when ec17dfc introduced
calling newItem->setKey() inside StorageHandler::addItem().  Anyway,
according to the documentation of StoragePlugin::addItems(), the
implementation was wrong since the initial commit. Also see 7b8c432
(removing the aLocalKey argument -- probably wrong conflict resolution)
and c4ce210 (adding aLocalKey again).

Relates NEMO#664

tests_meego/Mock.h

@@ -111,6 +111,8 @@ class MockStorage: public StoragePlugin {
         iFormats.setPreferredTx(format);
         iFormats.rx().append(format);
         iFormats.tx().append(format);
+
+        iIdCounter = 0;
     }
 
     virtual ~MockStorage() {}
@@ -198,7 +200,7 @@ class MockStorage: public StoragePlugin {
 
     virtual SyncItem* newItem()
     {
-        return new MockSyncItem( "1" );
+        return new MockSyncItem( "" );
     }
 
     virtual SyncItem* getSyncItem( const SyncItemKey& aKey )
@@ -228,6 +230,7 @@ class MockStorage: public StoragePlugin {
         QList<StoragePluginStatus> results;
 
         for( int i = 0; i < aItems.count(); ++i ) {
+            aItems[i]->setKey( QString("MockStorage-item-%1").arg( ++iIdCounter ) );
             results.append( STATUS_OK );
         }
 
@@ -267,7 +270,7 @@ class MockStorage: public StoragePlugin {
     QString                     iSourceURI;
     qint64                      iMaxObjSize;
     StorageContentFormatInfo    iFormats;
-
+    int                         iIdCounter;
 };
 
 class MockTransport : public DataSync::Transport {