Provide a way to automatically treat all async tests as trio tests

[njsmith]

We don't want to do this by default, because then we get into the situation where all the different pytest-{asyncio,trio,curio,...} plugins fight over which tests are theirs. And we want to preserve the option to mark individual tests with @pytest.mark.trio, for projects that want to use multiple pytest-* plugins at the same time.

But for the common case of a project that uses only trio, there should be some way to opt-in to automatic detection of async tests as trio tests.

An option in pytest.ini is one obvious thing. We should also give a way for conftest.py to opt-in directly, for the use of projects that ship their tests along with their source code but can't easily ship a pytest.ini.

What would be really neat was if we could limit the conftest.py opt-in to the subdirectory that conftest.py applies to. I'm not sure if there's a way to do that.

[njsmith]

I guess one option for marking things in conftest would be to tell people to write

from pytest_trio.automark import *

where pytest_trio/automark.py contains a plugin hook that sniffs test to see if they're async, and if so then automatically applies the trio marker.

This is evil and magical: the idea is to exploit pytest's magic where every conftest.py file is automatically treated as a pytest plugin that's applied only to the tests inside the same directory as that conftest.py. And the effects of a pytest plugin are determined by the contents of its namespace. So if we import a hook function, then that hook function becomes part of this "conftest plugin", and is applied where-ever the conftest is in scope.