Test using StableRNGs

[AsafManela]

Starting with Julia 1.5 random number generators are not guaranteed to provide the same sequence of numbers for a given seed (see here).

This PR replaced the global rng used in tests with a StableRNG called testrng and uses it everywhere.
Turns out that was not enough because Lasso.jl has a RandomCoefficientIterator that uses its own RNG.
So to get deterministic test, I also add the ability to specify an rng in the call to fit(...; rng=StableRNG(123)).

Tests now seem to pass on 1.5.
Closes #48

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[AsafManela]

Hi @andreasnoack, do you have time to review this one or should I just merge?