Fix test_compute_relative_contrast_with_random_data()

[arrayka]

Why

• The test_compute_relative_contrast_with_random_data() test still fails intermittently because average_squared_distance() internally creates its own RNG using random::create_rnd().
• In test mode, we expect random::create_rnd() to produce a deterministic, seeded RNG. However, the function is compiled in dev mode rather than test mode, so it doesn’t behave deterministically, leading to flaky test behavior.

What

• Pass an RNG instance into average_squared_distance() so the caller (including tests) can inject a deterministic RNG.
• [Optional] Update random::create_rnd() so that when compiled with feature = "testing", it returns a seeded, deterministic RNG.

Now, multiple runs of the test produce the same output:

Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035
Mean relative contrast: 1.1806035