Add functional test for IPC interface

[kevkevinpal]

mentioned in: bitcoin#33201

"re-ACK db52550

I checked again that the test isn't skipped on the machines where we enable it. It's skipped in the test each commit job:

243/273 - interface_ipc.py skipped (capnp module not available.)

That's for the last commit it runs, 1b69f79 ci: enable IPC tests in CI.

I would be fine with leaving that for a followup." - Sjors

[Sjors]

That test does run now, see my last comment on the PR.

[kevkevinpal]

That test does run now, see my last comment on the PR.

Awesome closing issue