Fix constructor parameters in tst_QMediaPlaylist.

Fix warning:
tst_qmediaplaylist.cpp: In member function 'void tst_QMediaPlaylist::mediaPlayListControl()':
tst_qmediaplaylist.cpp:1221:49: warning: the address of 'parent' will always evaluate as 'true' [-Waddress]

Change-Id: I46391550d07a8f58442269d0e5eae418258adbff
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>