tests/unit: cleanups for test-io-channel-command

tests/unit: cleanups for test-io-channel-command

This test is hanging under heavy load when the two socats race while
trying to create the socket. I've tried various approaches to avoid
the race but it seems "creat=0" won't stop socat trying to create a
pipe if it executes first. In the end I just use a small sleep which
seems to be reliable enough on the load situations I've tried.

While I was there I also properly created a tmpdir for the socket to
live in which is cleaned up at the end of the test.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Message-Id: <20221027183637.2772968-30-alex.bennee@linaro.org>