[PR #12394/24ed3b34 backport][3.14] docs: mention fake server testing pattern (#12396)

**This is a backport of PR #12394 as merged into master
(24ed3b3).**

Fixes #980.

Co-authored-by: nightcityblade <[email protected]>

Author: patchback[bot]

docs/testing.rst

@@ -490,6 +490,12 @@ basis, the TestClient object can be used directly::
 A full list of the utilities provided can be found at the
 :mod:`api reference <aiohttp.test_utils>`
 
+For end-to-end client code that talks to an external service, it is
+recommended to run a small fake server rather than patching private aiohttp
+internals. The ``examples/fake_server.py`` demo shows such an approach: start
+a local :class:`~aiohttp.web.Application`, point a custom resolver at it, and
+exercise the client against that controlled endpoint.
+
 
 Testing API Reference
 ---------------------