tests: fix problems introduced by the audio mixing.

	+ backgroundsource : we now have an adder in the composition.
	+ layer : comment out one check that caused assertions.
	+ effects : use a regular GESLayer.
audiomixing: various fixes in response to review.

	+ base-effect / uri-source / test-utils : Make pspec functions internal.
	+ smart-adder : reordering of declarations.
	+ uri-source : call no_more_pads after the pad has been added to the bin.
	+ uri-source : Only expose volume and mute for audio track types.
	+ uri-source : function renaming.