rust-discovery: optionally don't randomize at all, for tests As seen from Python, this is a new `randomize` kwarg in init of the discovery object. It replaces random picking by some arbitrary yet deterministic strategy. This is the same as what test-setdiscovery.t does, with the added benefit to be usable both in Python and Rust implementations. Differential Revision: https://phab.mercurial-scm.org/D6426