Edward Hervey authored Oct 10, 2019 and Tim-Philipp Müller committed Oct 10, 2019

When checking the behaviour of live seeking on audiomixer or
adder we don't *really* need real audio devices. audiotestsrc
in live mode is enough to test the behaviour of those elements.

Also avoids people repeatedly wasting hours trying to figure out
whether that failing behaviour is due to their code or not.