test: camerabin: Fix buffer size calculation

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: https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/998
which make us negoaite 1x1 instead of 16x16 in this test.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2297>