test: replace echo with socat

The original version of the test used netcat along with a standard
AF_UNIX socket, which caused issues across different netcat
implementations. The AF_UNIX socket was then replaced by a FIFO with a
simple echo, which, however, suffers from the same issue (some echo
implementations don't check if the write() was successful).

Let's revert back to the AF_UNIX socket, but replace netcat with socat,
which, hopefully, resolves the main issue.

Relevant commit: 9b45c2bf