tests: use color_float rgb[] alias more

Iterate over rgb[] array instead of repeating the code for .r, .g and
.b.

Also in process_pipeline_comparison() f_max_err variable is dropped
since it was not used much.

This should make the code easier to read.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>