QCheck int shrinker produces duplicates

[jmid]

As part of #235 I was reminded of the following - which we should address and track separately:

The duplicate 0 testing of the improved int shrinker is unsatisfying, but so is the duplicate output of the current int shrinker: https://github.com/c-cube/qcheck/blob/824dafb111961ec16bc3526173ad004d45820632/test/core/QCheck_unit_tests.ml#L20-L24

Originally posted by @jmid in https://github.com/c-cube/qcheck/issues/235#issuecomment-1103781517