tests: Remove useless mutex
tests: Remove useless mutex

pitivi/gst-editing-services,pitivi/gst-editing-services,pitivi/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/ges,cfoch/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,cfoch/gst-editing-services