Constant TEST_REPLICATE_OFFSET 

pub const TEST_REPLICATE_OFFSET: u32 = _; // 2_147_483_648u32

The offset added to a test replicate’s index so the test seed stream is disjoint from the tune stream.

Tune replicate j uses Stream::Replicate(j); test replicate j uses Stream::Replicate(TEST_REPLICATE_OFFSET + j). As long as the tune fan width stays below 2^31 (it always does — it is a handful of seeds) the two streams can never overlap, so a configuration is never tested on a seed it was tuned on. Part of the stability commitment.