make containers.testlib public

[c-cube]

There's a testlib sub-library which is basically a replacement for qtest. It's not inline, but it tries to make it as easy as possible to write more tests (e.g t_list.ml), so the statements are similar to qtest's, with a mix of unit tests and random tests.

I'm finding myself adding it to other projects to get rid of qtest, so maybe it should just be a fixture of containers? or a package containers-testlib perhaps?

I'd love to hear people's opinions on that.