Replace raw nc usage in statschannel system test with curl
As we already require nc for doth system test, replace the nc usage in the statschannel test with curl that actually understands the HTTP/1.1 protocol in a hope it will make the statschannel truncated test failure to go away, because it waits for the the requests to complete before sending next one.
Closes #3270 (closed)
Edited by Ondřej Surý