tests/unit: make test-io-channel-command work on win32

tests/unit: make test-io-channel-command work on win32

This has been tested under msys2 & windows 11. I haven't tried to make
it work with other environments yet, but that should be enough to
validate the channel-command implementation anyway.

Here are the changes:
- drop tests/ from fifo/pipe path, to avoid directory issues
- use g_find_program() to lookup the socat executable (otherwise we
would need to change ChanneCommand to use G_SPAWN_SEARCH_PATH, and deal
with missing socat differently)
- skip the "echo" test when socat is missing as well

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221006113657.2656108-7-marcandre.lureau@redhat.com>