Move pytest-trio's hook for deterministic Hypothesis tests upstream into Trio

[Zac-HD]

pytest-trio does two things for property-based tests with Hypothesis:

1. Inserts the async-to-sync wrapper logic. pytest-trio is clearly the right place to do that.
2. Registers Trio's internal Random instance so that the scheduler is deterministic and Hypothesis can replay failing examples (see Deterministic scheduling via Hypothesis pytest-trio#73 and Support deterministic scheduling #890).

At the time, pytest-trio was the right place to do (2), but not anymore: since HypothesisWorks/hypothesis#2556, it's possible for Trio to ship a Hypothesis plugin which handles the registration if-and-only-if Hypothesis is imported. Here are the docs.

This is useful because it works even for users without pytest-trio installed or enabled (c.f. HypothesisWorks/hypothesis#3940), e.g. because they're using unittest or running a test function by hand in some debugging loop or the test function calls trio.run() internally, and it will allow pytest-trio to avoid importing Hypothesis if it wasn't otherwise going to be used (which is nice for performance).