Make test_rng randomized by default in std

[Pratyush]

Description

Makes test_rng use a randomized seed by default when std is set. The benefit of this is that our "fuzzing" tests actually explore the search space now. Longer term we should also introduce a fuzzing harness to explicitly perform the search. To recover the old deterministic behaviour, the user can set env DETERMINISTIC_TEST_RNG=1 when running tests.

Also makes the output of test_rng opaque to the user, by having it return impl Rng instead.

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