[amarok] src: Prevent hang in testmetamultitrack.

[Matěj Laitl]

On 12. 7. 2012 Matěj Laitl wrote:
> Because I'm the author of the original change, I'll take responsibility to
> fix  the test that is broken by it.

Okay, it is done in http://git.reviewboard.kde.org/r/105524/ - please review. 
It removes the mutex altogether and it fixes a couple more things including 
testm3uplaylist. (at least for me)

	Matěj