Support `trio_run = trio_asyncio` ini option

[jmehnle]

trio_asyncio as another mode of the trio_run ini file option allows us to run tests with an asyncio loop implicitly available to all async tests. This way one doesn't need to explicitly call trio_asyncio.open_loop() from tests or from a fixture (https://github.com/python-trio/pytest-trio/issues/71).

This PR is inspired by qtrio support (https://github.com/python-trio/pytest-trio/pull/105).