tests: replace mat2XYZ

The primaries and the white point are the fundamental definition of the
color spaces in these tests. Instead of hard-coding mat2XYZ, use
LittleCMS to derive the result from the fundamental definition.

This removes derived hard-coded constants, which is a benefit in itself.
How these constants were originally produced was not mentioned in
0c5860fa but I was able to reproduce
them with python3:

import colour
import numpy as np
x = colour.RGB_COLOURSPACES['sRGB']
w_d50 = np.array([0.34567, 0.35850])
print(x.chromatically_adapt(w_d50, 'D50', 'Bradford'))

It's identical to 3-4 decimals of the hardcoded values, and also for
Adobe RGB. I printed the LittleCMS generated values as well, and they
are the same as with python up to roughly 4 decimals.

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