ges:tests:validate: Add tests disabling mixing in the default testsuite

Submitted by Thibault Saunier @thiblahute

Link to original bug (#765935)

Description

We should test that GES is useable even when disable mixing, meaning that
Gst elements, and in particular demuxers properly handle seqnums as NLE
intensively relies on them to thread safely know when a sub pipeline is
EOS.