tests/channel-helper: set blocking in main thread

tests/channel-helper: set blocking in main thread

The /io/channel/command/echo tests run the reader side and the writer
side with the same underlying command channel. Setting the blocking mode
of the fd/handles while the other end is already reading/writing may
create issues (deadlock in win32 when earlier attempt of this series
were using SetNamedPipeHandleState). Let's just do it before spawning
the threads to avoid further concurrency issues.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221006113657.2656108-4-marcandre.lureau@redhat.com>