Support stdlib unittest

[logileifs]

I've noticed that the recommended way to write tests for curio programs is to use pytest and install the curio plugin. I prefer to just stick to the unittest library from the standard library so to be able to run curio tests without pytest I created a CurioTestCase class which mimics the IsolatedAsyncioTestCase included with unittest. It works with both pytest and pure unittest. I already created a pull request but I thought it would be good to open an issue as well for visibility/discussion.

[dabeaz]

There's a recommended way to write tests? Where is this documented?

[logileifs]

Well maybe not recommended but judging by the issues (https://github.com/dabeaz/curio/issues/330 for example) it seems to be the standard way to write tests for curio programs.

[dabeaz]

I don't have any official recommendation regarding the testing of Curio programs. The test suite uses pytest, but it doesn't rely upon any plugins to my knowledge.

[logileifs]

Anyway, I think this would only make curio easier and more approachable. If you don't agree then feel free to close this.