tests: add socat dependency for tests

tests: add socat dependency for tests

We only use it for test-io-channel-command at the moment.
Unfortunately bringing socat into CI exposed an existing bug in the
test-io-channel-command unit test so we disabled it for MacOS in the
previous patch.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230228190653.1602033-3-alex.bennee@linaro.org>