Please allow keyword arguments of deterministic scheduling and random-value generation

Deterministic scheduling is a very desirable characteristic for user code testing where external interactions can be mocked-up. In my view, this is one of the major advantages of trio over asyncio. Deterministic scheduling is controlled by the following two lines in core/_run.py:

_ALLOW_DETERMINISTIC_SCHEDULING: FinalT = False
_r = random.Random()

Right now these are monkey-patched by pytest-trio, but not all user code uses pytest-trio to create test runners (for example, anyio). Downstream users are thus left to monkey-patch the runner factory themselves.

It would be nice if Trio's constructor had these as keyword arguments. I think separate arguments would be best, because I can think of cases where one might wish to control random-value generation without incurring the performance hit of deterministic scheduling.

Start in core/_run.py by reading _ALLOW_DETERMINISTIC_SCHEDULING and _r, then trace the Trio runner or constructor entry point that controls them. Confirm how pytest-trio currently monkey-patches these values and define completion as independently configurable keyword arguments for deterministic scheduling and random-value generation, without requiring downstream monkey-patching.