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.