Test case executor for WSGI apps

[Stranger6667]

Similar to what Flask has for testing - we don't have to use requests, we can work with the application directly. For this, we need to make this component interchangeable and write a one that will be able to execute schemathesis tests against an arbitrary WSGI app.

In general, this approach should be much faster than using a real network. The executor API design is up to discussion (feel free to propose any).

The main point of the issue is to simplify Schemathesis adoption - it is easier and faster to use a WSGI instance in certain frameworks like Flask or Django.

The implementation could be adopted from existing test clients, for example from Flask.

https://github.com/pallets/flask/blob/master/src/flask/testing.py#L115

Or, maybe we can use werkzeug directly - https://github.com/pallets/werkzeug/blob/master/src/werkzeug/test.py#L768

PEP: https://www.python.org/dev/peps/pep-3333/