Support global deterministic seed

[cmeeren]

Not sure if this is feasible, but I'll try:

I like deterministic builds. Being able to easily re-build and re-deploy a previous build has saved me many times. (Let's keep aside the roll-forward vs. roll-back debate. It's irrelevant anyway; the below also applies to a roll-forward.)

I have at times been thwarted by Hedgehog properties failing on CI builds where it did not fail locally, and where re-running makes it pass. Or vice versa; failing locally, but unexpectedly passing on CI. The reason is that a rare combination of generated values surfaces a bug (most often this is in the test/generator code, though occasionally it is in the production code). This combination does not occur on every run.

I would like to bring determinism back to my test suite, by having Hedgehog use the same seed every time, so to speak. So that, sure, when it runs a property 100 times, the values are seemingly random, but subsequent runs of the test produces the same values for each respective iteration. I want to be able to run a test as many times as I want, and never get different results, for the aforementioned reasons.

The only conceptual drawback I see to this is that making the seed random on every run increases the chance of catching a bug. But personally, that is not a way I want to catch a bug. It indicates a poorly written property that does not sufficiently try to reproduce relevant edge cases. I value deterministic builds more. If I want increased confidence in a property, I just increase the property's test count.