tests: add functional tests for IPC interface

Co-Authored-By: ismaelsadeeq <ask4ismailsadiq@gmail.com>
Co-Authored-By: ryanofsky <ryan@ofsky.org>
Co-Authored-By: TheCharlatan <seb.kung@gmail.com>
Co-Authored-By: Sjors Provoost <sjors@sprovoost.nl>

test/README.md

@@ -34,6 +34,22 @@ The ZMQ functional test requires a python ZMQ library. To install it:
 - on Unix, run `sudo apt-get install python3-zmq`
 - on mac OS, run `pip3 install pyzmq`
 
+The IPC functional test requires a python IPC library. `pip3 install pycapnp` may work, but if not, install it from source:
+
+```sh
+git clone -b v2.1.0 https://github.com/capnproto/pycapnp
+pip3 install ./pycapnp
+```
+
+If that does not work, try adding `-C force-bundled-libcapnp=True` to the `pip` command.
+Depending on the system, it may be necessary to install and run in a venv:
+
+```sh
+python -m venv venv
+git clone -b v2.1.0 https://github.com/capnproto/pycapnp
+venv/bin/pip3 install ./pycapnp -C force-bundled-libcapnp=True
+venv/bin/python3 build/test/functional/interface_ipc.py
+```
 
 On Windows the `PYTHONUTF8` environment variable must be set to 1: