Add scope_client parameter to TestClient

[pythonweb2]

Summary

A breaking change was made when the TestClient's scope["client"] was set to None. In cases where an application uses the scope["client"] in places like custom middleware, logging, etc, upgrading to 0.35 becomes a very hard task. Testing middleware/logging/business logic that relies on the client being populated becomes hard if not impossible. Middleware that worked before breaks, and now we need to add checks in application code to disable certain things based on whether or not the test suite is running.

This PR adds the ability for users to specify what the scope["client"] should be when using the test client. This way, when using fixtures in a framework like pytest (which is the recommended setup), a one line change is all that is required.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[pythonweb2]

I made it so instead of reverting the change, there is a new option to keep the old functionality, which should be less controversial (if some have already change their code for the 0.35.0 release). Would this be OK to add?

@Kludex

[Kludex]

Let's discuss it first.