Add unit and expect tests

[jmid]

This is the second episode of nicking the good non-shrink-log parts from #172, #174, and #176.

Highlights:

• by renaming expect-files we can utilize dune's expect test support and simplify the dune files
• tests of check_exn
• unit-tests for QCheck, corresponding to those from QCheck2
• a tree shrinker test for QCheck, corresponding to the one from QCheck2
• a bunch of tuple and bind-tests, both positive, negative, and with statistics

I see this last item as supplementing the tup tests from @vch9. I want to collect our tuple tests, but in order to keep the noise down I suggest we do so in a separate PR.

The PR is probably best read one commit after another. None of this should be controversial though.
Objections or a quick glance and thumbs-up would be appreciated, as I would like to get the last tests in play before they bit rot further...

[jmid]

I'll merge these tests shortly unless I hear an outcry of objections... 😀