Deprecate `testtools` support

At the time of writing, our documentation might [give an impression](https://github.com/falconry/falcon/discussions/2155) that using `testtools` is the preferred way to write tests for a Falcon app.

I haven't done any research or survey in the area, but just subjectively it feels that the most popular testing frameworks in 2023 are `pytest` followed by the stdlib's `unittest` (both are also well supported by Falcon).
Moreover, IIRC I had to exclude `testtools` from our test dependencies when adding Python 3.10 CI gates, since it wasn't 3.10-ready by the time. So it's becoming a maintenance burden, too.

OTOH, it seems that there has been a new [`testtools` release](https://pypi.org/project/testtools/), and the project is still maintained. So after all maybe it doesn't hurt to keep the integration?

Even if we decide to keep `testtools`, we should IMHO reword the docs not to give the impression it is the preferred way, and link to our tutorial for using `pytest` in the same paragraph.

Start by reading the linked discussion #2155 and locating the documentation that presents testtools as the preferred testing approach. Determine whether the project will remove testtools support or retain it with revised wording and a link to the pytest tutorial; done means that decision is reflected consistently in the documentation and integration.

Written by the indexing model from the issue text.