Fix tests on 32bit arch

[jmid]

I've been preparing a 0.19 release - but the thorough opam CI pointed out a couple of issues,
including that the expect tests fail on 32-bit architectures.

This PR is thus a quick-and-dirty fix to solve that - thereby fixing #152.

• On 32-bit the distribution of Random will be different. This is solved by including separate 32/64-bit expected outputs.
• A few unit tests were comparing shrink counts - which also differ! Those tests are rewritten to no longer test the shrink count.