Add support for async steps & scenarios

[amadeuszhercog-silvair]

Relates to #223

This pull request adds support for asynchronous steps and scenario functions. As i wrote in README.rst user will be able to use async steps and scenarios out of the box like this:

@pytest.mark.asyncio
@scenario('test.feature', 'Launching scenario function')
def test_launching_scenario_function():
    pass

@given("i have async step")
async def async_given():
    pass

@when("i do async step")
async def async_when():
    pass

@then("i should have async step")
async def async_then():
    pass

Explanation about tests

I've put all tests that use asyncio into tests/asyncio. Their structure is following:

• test_async_given_returns_value.py - tests checking that async given is a fixture or it can shadow other fixture,
• test_async_scenario_function.py - tests for scenario function,
• test_async_steps.py - tests for async steps,
• test_launching_app_in_background.py - (bigger than above ones) test that will launch dummy application based on asyncio in event_loop.create_task and will interact with it by http protocol. It checks that app is still running between steps and it can respond.
• dummy_app.py - dummy app from previous point + flask app with which is "gateway" between dummy app and tests.

Data flow of tests based on dummy app looks like this:

test function -> flask server -> dummy app -> flask server -> test function

---

That PR will require dropping Python 2.7 support before. But since Python 2.7 is no longer officially supported anyway I don't think it would be a problem.

[yanlend]

I don't think the test with the app is worth to add all those testing requirements.

Otherwise, I like the PR. I think adding a native support for asyncio is useful!

[mikeoconnor0308]

Are there any plans to look at getting this merged?

[youtux]

Hi @amadeuszhercog-silvair, please check my comment about trying to use a different approach for this issue.

It would be better to have async code compatibility without being tied to a specific pytest plugin.