Fix some memory leaks.

Adjusted a unit test or two to make leaks easier to find, and then fixed
a few leaks.

Change-Id: I47a20df8de7cac113d34a2015f76a65ab957dedd
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

tests/auto/unit/qmediaplayer/tst_qmediaplayer.cpp

@@ -891,6 +891,8 @@ void tst_QMediaPlayer::testPlaylist()
         QCOMPARE(player->media(), content2);
     }
 
+    delete playlist;
+    delete playlist2;
 }
 
 void tst_QMediaPlayer::testPlayerFlags()
@@ -1081,6 +1083,8 @@ void tst_QMediaPlayer::testPositionPropertyWatch()
 
     positionSpy.clear();
     QTRY_COMPARE(positionSpy.count(), 0);
+
+    delete playlist;
 }
 
 void tst_QMediaPlayer::debugEnums()