Avoid using set.seed() in unit tests and examples

[PietrH]

As I understand the documentation, once you call set.seed() the seed is set for the session, even if future function calls don't specify a seed. So to avoid overwriting the seed of users, and to have the package behaviour as predictable as possible, we should avoid doing this.

See this section of the R packaging guide; https://r-pkgs.org/code.html#sec-code-r-landscape

Maybe using withr? I'm not sure yet, I'm splitting this off from #36 as it's no longer in use within the functions of this package, and thus now less urgent.

[wlangera]

fixed in #98