Extend expect tests to OCaml 5.0

[jmid]

The forthcoming OCaml 5.0 comes with a new splittable RNG in the Random module. As a consequence the distributions of QCheck's generators change, which means that the existing expect tests will fail (like they have done for 32-bit architectures #152).

This should be fixed, preferably in a sustainable way to also allow testing on 32-bits (initial support in #250).