tests: enforce I420 format
Test was not enforcing a video format on videotestsrc. I420 was picked as it was the first format in GST_VIDEO_FORMATS_ALL which will no longer be true (gst-plugins-base!689 (merged)).
Test was not enforcing a video format on videotestsrc. I420 was picked as it was the first format in GST_VIDEO_FORMATS_ALL which will no longer be true (gst-plugins-base!689 (merged)).