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This should solve the race condition when the socket file is created,
but connect to it does not work yet, because listen() was not called on
it yet. Really connecting ensures it is possible to connect.

Place the socket higher in the hierarchy (to the top-level build
directory) to have a shorter path.

It should be more reliable to ensure all messages arrived.

The success cases of sending, receiving, subscribing, etc.

Provide a test suite to launch and stop msgq from tests as separate
process. It'll be used to test it works correctly, by connecting to it
from the tests.