Enable "asyncio debug mode" for default event loop

[p-rogalski]

The Python asyncio library has a debug mode, which is supposed to ease developing: https://docs.python.org/3/library/asyncio-dev.html#debug-mode

What about enabling this in pytest-asyncio for the default event loop? This could be made configurable via a config option.

[asvetlov]

I expect a lot of Executing <...> took 0.103 seconds warnings at least.

Adding a config option sound like a great idea though.

[seifertm]

I would also support a PR that adds a config option (+command line option) to enable the debug mode.

Until then, you can use some of the existing means to enable debug mode, such as setting the PYTHONASYNCIODEBUG environment variable.