[Bug 700600] videomixer: Fix races in caps negotiation and re enable the unit test

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=700600
  GStreamer | gst-plugins-good | 1.x

Thibault Saunier <thibault.saunier> changed:

           What    |Removed                     |Added
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                 CC|                            |thibault.saunier at collabora.
                   |                            |com

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