test: Clarify that Python UTF-8 mode is the default today for most systems

It will likely be the default for all systems, starting with Python
3.15, according to https://peps.python.org/pep-0686/#abstract.

It is hard to find a system other than Windows that has it not enabled
today. Nonetheless, Bitcoin Core requires UTF-8 in scripts and normally
enforces it via LC_ALL=C.UTF-8 or PYTHONUTF8=1.

test/README.md

@@ -51,6 +51,8 @@ venv/bin/pip3 install ./pycapnp -C force-bundled-libcapnp=True
 venv/bin/python3 build/test/functional/interface_ipc.py
 ```
 
+The functional tests assume Python UTF-8 Mode, which is the default on most
+systems.
 On Windows the `PYTHONUTF8` environment variable must be set to 1:
 
 ```cmd