Add missing file - buildbot is useful!

tests/mock_playlistitem.cpp

@@ -0,0 +1,25 @@
+/* This file is part of Clementine.
+
+   Clementine is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   Clementine is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with Clementine.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "mock_playlistitem.h"
+
+using ::testing::_;
+using ::testing::Return;
+
+MockPlaylistItem::MockPlaylistItem() {
+  EXPECT_CALL(*this, Save(_))
+      .WillRepeatedly(Return());
+}