Add tests

[sir4ur0n]

Ironically there aren't tests for QCheck, which makes it uncomfortable/non-trivial to test the features (mainly, generators and shrinkers).

This could be both unit tests (e.g. "Shrinking 100 towards 0 gives 0, 50, 75, etc.") and PBTs (e.g. "int_range a b only gives values between a and b").

This would also help reproducing bugs in QCheck issues

[c-cube]

Yep 😅 . That'd be useful.

[jmid]

Closing as we now have a collection of expect tests and unit tests, e.g., from #234 and #237