test: use math/rand/v2

[Juneezee]

We upgraded our minimum Go version in PR #39, which allows us to use the math/rand/v2 package, a more 'correct' implementation compared to math/rand.

One of your papers was mentioned in the blog post: https://go.dev/blog/randv2 😉

In 2018, Daniel Lemire found an algorithm that avoids the divisions nearly all the time (see also his 2019 blog post). In math/rand, adopting Lemire’s algorithm would make Intn(1000) 20-30% faster, but we can’t: the faster algorithm generates different values than the standard conversion, breaking repeatability.

[lemire]

Nice. Thanks.