Add testclient test for domain restricted cookies

[nidico]

Summary

This adds a test for #2152 which creates responses with domain restricted cookies for different domains.

The test with the testserver domain parametrization succeeds in Python 3.11 but fails on Python 3.10 and below.

Note that this PR just adds a partially failing test, but doesn't resolve #2152 (e.g. by changing documentation or changing the test client default base_url).

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.

[Kludex]

I'm going to merge this for the time being. I understand that the problem is not solved, but at least documented on the test.

Let's see if we can solve it as a next step.

Thanks @nidico :)