Test CrossHair backend in the CI

[JonathanPlasse]

get hypothesis-crosshair running in our CI

Is this what you had in mind for #3914

[Zac-HD]

Not quite - adding the tox + CI task yes, but as well as adding a new set of tests I want to try running all the existing tests using crosshair. I think that will involve setting up a new profile in hypothesis-python/tests/common/setup.py, then enabling it via an environment variable, and finally running the usual cover/nocover/niche tests.

The point of this exercise is not that all the tests will work - some of the internals tests shouldn't, on an alternative backend. The point is to discover various problems where we want to fail gracefully and currently just fail. Depending on how many and how serious they are, we'll decide what to run in CI and what we'll just revisit later 🙂

[Zac-HD]

I've ultimately cherry-picked these changes into #3935, but left them disabled in CI due to the current high failure rate.

Thanks again for working on this!