test: camerabin: Fix buffer size calculation
We were assunming that GStreamer size for RGB (24bit packed) data was width x height x 3, but GStreamer defaults to specific alignment. Use GstVideoInfo API in order to obtain the buffer size.
This fixes failure seen when trying to merge: gst-plugins-good!998 (merged) which make us negoaite 1x1 instead of 16x16 in this test.