Align with curio

[geronimo-iia]

Hello, thanks for your work ! I've learned few things with pytest cause of you :)

Here, I purpose you few change:

• align kernel call to last released curio version (no add_task), use curio.meta to test coroutine
• add a kernel finalizer on fixture with a session scope
• raise root cause exception on error (rewrote of https://github.com/johnnoone/pytest-curio/pull/2/files)

Tested with success on pynng (curio integration is a work in progress)