Extend migration guide with details about Connexion test_client for testing and tox

[chrisinmtown]

Description

Please extend the migration guide with some details about the required change from getting a client by calling the Flask app's test_client() function, to getting a client by calling the Connexion app's test_client() function.

Expected behaviour

Migration guide also explains how to migrate tests. Ideally, one of the example projects includes a tox.ini file and a test case to demonstrate how to test a Connexion v3 app.

Actual behaviour

The testing page is helpful for pointing people to the test_client() function, but you have to go looking for it, since the migration guide is silent.

https://connexion.readthedocs.io/en/latest/testing.html